Diseases and immunity Revision Notes

A pathogen is any organism capable of causing disease in another living organism. Unlike harmless or beneficial microorganisms, pathogens invade the body and interfere with normal biological processes, leading to illness. They are not limited to one type of organism but include several groups such as bacteria, viruses, fungi, and parasites. Understanding pathogens is essential for identifying causes of diseases and developing methods to control or prevent infections.

• Pathogens are disease-causing organisms.
• They can infect humans, animals, and plants.
• They disrupt normal body functions leading to symptoms of disease.

Pathogens belong to different biological groups, each with unique characteristics. Bacteria are single-celled organisms that can reproduce rapidly inside the body. Viruses are much smaller and require a host cell to multiply. Fungi include organisms like yeasts and moulds that can infect skin or internal tissues. Parasites are organisms that live on or inside a host, often causing harm over time. Recognising these types helps in diagnosing diseases and choosing appropriate treatments.

• Bacteria: single-celled, reproduce independently.
• Viruses: non-cellular, need host cells to reproduce.
• Fungi: can be single or multicellular, cause infections like athlete's foot.
• Parasites: live on/in hosts, e.g., tapeworms.

Pathogens cause disease by entering the body and multiplying, which can damage tissues and organs. For example, bacteria may release toxins that harm cells, while viruses hijack host cells to produce more viruses, often killing the cells in the process. Fungal infections can cause inflammation and tissue damage, and parasites may consume nutrients or cause physical damage. The body's normal functions are disrupted, leading to symptoms such as fever, pain, or fatigue.

• Pathogens invade and multiply inside the host.
• Bacteria may produce toxins harmful to the body.
• Viruses replicate inside host cells, causing cell damage.
• Fungi and parasites damage tissues or consume nutrients.

Different pathogens cause specific diseases. For instance, the bacterium *Streptococcus* causes strep throat, a viral pathogen like influenza virus causes the flu, fungi such as *Candida* species cause thrush, and parasites like *Plasmodium* cause malaria. These examples illustrate the diversity of pathogens and the diseases they cause, highlighting the need for targeted medical interventions.

• Bacterial disease: Tuberculosis caused by *Mycobacterium tuberculosis*.
• Viral disease: Measles caused by measles virus.
• Fungal disease: Athlete's foot caused by dermatophyte fungi.
• Parasitic disease: Malaria caused by *Plasmodium* species.

Knowing what pathogens are and how they cause disease is fundamental for public health. It guides the development of vaccines, antibiotics, antifungal and antiparasitic drugs. It also informs hygiene practices and infection control measures to prevent disease spread. For example, recognising that viruses need host cells to reproduce explains why antibiotics do not work against viral infections, emphasising the need for vaccines or antiviral drugs instead.

• Understanding pathogens aids in disease prevention and treatment.
• Different pathogens require different medical approaches.
• Knowledge supports public health measures to control infections.

A transmissible disease is an illness caused by a pathogen that can be passed from one living host to another. The pathogen is the microorganism or agent responsible for causing the disease, such as bacteria, viruses, fungi, or parasites. The key feature of transmissible diseases is that the pathogen moves between individuals, enabling the disease to spread within populations.

For example, the common cold is a transmissible disease because the virus causing it can be transferred from one person to another through sneezing or touching contaminated surfaces. In contrast, diseases caused by genetic defects or environmental factors are not transmissible because they do not involve pathogen transfer between hosts.

• Transmissible diseases require a pathogen that can move between hosts.
• The pathogen causes infection in the new host after transfer.
• Not all diseases are transmissible; some are non-infectious.

Pathogens can be transferred from one host to another through various routes. Direct transfer involves physical contact between individuals, such as touching, kissing, or sexual contact. Indirect transfer occurs when pathogens are carried by objects, air, food, water, or other organisms called vectors.

For example, the bacterium causing tuberculosis spreads mainly through airborne droplets when an infected person coughs. Malaria is transmitted by mosquitoes acting as vectors, transferring the parasite from one human host to another. Understanding these transfer mechanisms is essential to interrupt the chain of infection.

• Direct contact: physical interaction spreads pathogens.
• Indirect contact: contaminated surfaces, food, or air can carry pathogens.
• Vectors: animals or insects can transfer pathogens between hosts.

The main ways pathogens spread include: direct contact, indirect contact, airborne transmission, and vector-borne transmission. Direct contact involves touching an infected person or their bodily fluids. Indirect contact includes touching contaminated objects like doorknobs or sharing utensils. Airborne transmission occurs when pathogens travel in droplets or dust particles suspended in the air. Vector-borne transmission involves organisms such as mosquitoes or ticks carrying pathogens.

For instance, influenza viruses spread through airborne droplets, while cholera bacteria spread through contaminated water. Recognising these modes helps target prevention strategies effectively.

• Direct contact: skin-to-skin or bodily fluids.
• Indirect contact: contaminated objects or surfaces.
• Airborne: droplets from coughs or sneezes.
• Vectors: insects or animals transmitting pathogens.

Knowing how transmissible diseases spread is crucial for controlling outbreaks and protecting public health. By identifying transmission routes, health authorities can recommend specific measures such as handwashing, vaccination, quarantine, or vector control to reduce infection rates.

For example, during a flu outbreak, promoting respiratory hygiene and vaccination can limit spread. In malaria-endemic areas, using insecticide-treated nets reduces mosquito bites and disease transmission. Effective prevention depends on interrupting the pathogen transfer between hosts.

• Transmission knowledge guides disease control measures.
• Prevention strategies target specific transmission routes.
• Interrupting pathogen transfer reduces infection rates.

Pathogens are microorganisms such as bacteria, viruses, fungi, or protozoa that cause disease in their hosts. For an infection to occur, these pathogens must move from one host to another. This movement is called transmission. Transmission routes determine how easily a disease can spread within a population and influence the measures needed to control outbreaks.

There are two main categories of transmission: direct and indirect. Each involves different mechanisms and examples, which are important to distinguish for effective disease prevention.

• Pathogens require a transmission route to infect new hosts.
• Transmission routes are broadly classified as direct or indirect.
• Understanding transmission helps in designing control measures.

Direct transmission occurs when pathogens pass straight from an infected individual to a susceptible person without an intermediate object or organism. This often involves physical contact or exchange of bodily fluids.

Examples include touching, kissing, sexual contact, or contact with blood. For instance, HIV is transmitted through direct contact with infected blood or sexual fluids. Similarly, diseases like Ebola spread through direct contact with the blood or bodily fluids of infected individuals.

Direct transmission can also occur through droplets expelled when an infected person coughs or sneezes, if these droplets land directly on another person’s mucous membranes (eyes, nose, mouth).

• Direct contact involves physical interaction between hosts.
• Blood and other bodily fluids are common media for direct transmission.
• Droplet transmission is a form of direct contact when droplets reach another person directly.
• Examples: HIV via blood, influenza via droplets.

Indirect transmission involves the transfer of pathogens via an intermediate object, organism, or environmental medium. The pathogen does not move directly from one host to another but uses a vehicle or vector.

Common indirect routes include contaminated surfaces (fomites), food, animals, and air.

For example, touching a doorknob contaminated with cold virus particles and then touching your face can lead to infection. Foodborne pathogens like Salmonella spread through contaminated food. Animals such as mosquitoes can act as vectors transmitting malaria parasites. Airborne transmission occurs when tiny pathogen-containing particles remain suspended in air and are inhaled, such as tuberculosis bacteria.

• Indirect transmission uses an intermediate carrier or medium.
• Contaminated surfaces can harbour pathogens for hours or days.
• Food can carry pathogens if improperly handled or cooked.
• Animals can be vectors transmitting diseases between hosts.
• Airborne transmission involves inhaling pathogen-laden particles suspended in air.

To clarify, consider the following examples: A person with the flu coughs directly onto another person’s face — this is direct droplet transmission. If the same person coughs onto a table, and another person touches the table then their mouth, this is indirect transmission via a contaminated surface.

Eating undercooked chicken contaminated with Salmonella bacteria is an example of indirect transmission through food. A mosquito bite transmitting malaria parasites is indirect transmission via an animal vector.

These examples highlight how the same pathogen can sometimes spread through multiple routes, complicating control efforts.

• Direct droplet transmission: flu virus from cough to face.
• Indirect surface transmission: flu virus on table to hand to mouth.
• Foodborne transmission: Salmonella in undercooked chicken.
• Vector transmission: malaria via mosquito bite.

Knowing how pathogens spread helps in designing effective prevention strategies. For direct transmission, measures include avoiding contact with infected individuals, using protective barriers like gloves or condoms, and isolating patients.

For indirect transmission, cleaning and disinfecting surfaces, proper food hygiene, controlling animal vectors, and improving ventilation to reduce airborne particles are key.

For example, handwashing reduces indirect transmission from surfaces, while mosquito nets prevent vector-borne diseases. Vaccination can also reduce the number of infected individuals, lowering transmission overall.

• Prevention depends on transmission route.
• Direct transmission control: avoid contact, use barriers, isolate cases.
• Indirect transmission control: hygiene, disinfection, vector control, ventilation.
• Understanding routes guides public health interventions.

The skin is the body's first line of defense against pathogens. It forms a tough, continuous outer layer that physically blocks microorganisms from entering the body. The outermost layer of skin, the epidermis, consists of dead cells that are difficult for pathogens to penetrate. Additionally, skin produces oils and sweat that contain antimicrobial substances, creating an environment hostile to many microbes.

For example, when you have a cut, the skin barrier is broken, increasing the risk of infection, which is why cleaning wounds is important.

• Skin is a tough, impermeable barrier preventing pathogen entry.
• Outer dead cell layer resists microbial invasion.
• Skin secretions contain antimicrobial chemicals.

Nasal hairs filter the air we breathe by trapping dust particles and microorganisms before they enter the lungs. This reduces the number of pathogens inhaled.

Mucus, a sticky fluid lining the respiratory tract, traps pathogens and dust particles. Tiny hair-like structures called cilia move the mucus upwards towards the throat, where it can be swallowed or expelled, removing trapped pathogens from the body.

For instance, when you have a cold, excess mucus production helps trap viruses and bacteria, aiding their removal.

• Nasal hairs trap airborne particles and microbes.
• Mucus captures pathogens in airways.
• Cilia move mucus to throat for removal.

The stomach produces hydrochloric acid, creating a highly acidic environment with a pH around 1.5 to 3.5. This acidity kills many pathogens ingested with food or drink, preventing them from infecting the intestines.

For example, bacteria like Salmonella are often destroyed by stomach acid, reducing the risk of food poisoning if the acid barrier is intact.

However, some pathogens can survive stomach acid, which is why other defenses are necessary.

• Stomach acid has a very low pH, killing many ingested pathogens.
• Acts as a chemical barrier in the digestive system.
• Reduces risk of infection from contaminated food.

If pathogens bypass physical and chemical barriers, white blood cells provide an internal defense. Different types of white blood cells identify, engulf, and destroy pathogens.

Phagocytes engulf pathogens by a process called phagocytosis, digesting them internally.

Lymphocytes produce antibodies that specifically target pathogens, marking them for destruction or neutralising toxins they produce. This immune response helps clear infections and provides immunity against future infections by the same pathogen.

• White blood cells detect and destroy invading pathogens.
• Phagocytes engulf and digest microbes.
• Lymphocytes produce antibodies targeting specific pathogens.

Viruses are unique biological entities that differ from living cells. They are much smaller than bacteria and cannot carry out life processes independently. Instead, viruses rely entirely on infecting host cells to reproduce and propagate.

Unlike cells, viruses do not have cellular structures such as cytoplasm or organelles. Their simplicity is defined by two main components: a protein coat and genetic material.

• Viruses are microscopic infectious agents.
• They lack cellular structures and independent metabolism.
• They require a host cell to reproduce.

The protein coat, also called the capsid, is a protective shell surrounding the virus's genetic material. It is made up of protein subunits called capsomeres, which assemble into specific shapes such as helical or icosahedral forms.

This coat protects the genetic material from damage and helps the virus attach to and enter host cells. The capsid's shape and surface proteins determine which cells the virus can infect, as these proteins interact with receptors on the host cell surface.

For example, the influenza virus has a capsid with surface proteins that bind specifically to respiratory tract cells.

• Protein coat is called the capsid.
• Composed of protein subunits called capsomeres.
• Protects genetic material from damage.
• Facilitates attachment to host cells.
• Determines host specificity through surface proteins.

Inside the capsid lies the virus's genetic material, which carries the instructions for making new viruses. This genetic material can be either DNA or RNA, but never both in the same virus.

DNA viruses store their genetic information in deoxyribonucleic acid, similar to cellular organisms, while RNA viruses use ribonucleic acid. The type of genetic material affects how the virus replicates inside the host cell.

For example, the common cold virus is an RNA virus, whereas the herpes simplex virus is a DNA virus.

• Genetic material can be DNA or RNA, not both.
• Contains instructions for virus replication.
• Type of nucleic acid influences replication method.
• Examples: RNA virus (common cold), DNA virus (herpes simplex).

Viruses cannot reproduce on their own because they lack the cellular machinery needed for metabolism and protein synthesis. Instead, they inject their genetic material into a host cell and hijack the cell's machinery to produce viral components.

The host cell then assembles these components into new virus particles, which can leave the cell to infect others. This dependence on host cells is a key characteristic distinguishing viruses from living organisms.

For example, bacteriophages infect bacteria and use bacterial enzymes to replicate their DNA and produce new viruses.

• Viruses lack metabolic and protein synthesis machinery.
• They rely on host cells to replicate.
• Host cell machinery produces viral components.
• New viruses assemble and exit to infect other cells.

In summary, viruses are characterised by a protective protein coat called a capsid and genetic material that can be DNA or RNA. Their inability to reproduce independently means they must infect host cells to replicate.

These features explain why viruses behave differently from bacteria and other microorganisms, influencing how diseases caused by viruses are treated and prevented.

• Protein coat (capsid) protects genetic material.
• Genetic material is either DNA or RNA.
• Viruses require host cells for replication.
• These features distinguish viruses from other microorganisms.

Clean water is essential for drinking, cooking, and washing. Without it, people are at high risk of contracting waterborne diseases such as cholera, typhoid, and dysentery, which are caused by pathogens like bacteria and protozoa present in contaminated water.

Providing access to clean water interrupts the transmission of these pathogens by removing the source of infection. For example, when a community uses a protected well or treated water supply, the incidence of diarrhoeal diseases significantly decreases.

In practical terms, water treatment methods such as filtration, chlorination, or boiling kill or remove pathogens, making water safe to consume. This reduces the ingestion of infectious agents and thus lowers disease spread.

• Waterborne diseases spread through contaminated water.
• Clean water removes pathogens from drinking sources.
• Water treatment methods kill or remove infectious agents.

Food can become contaminated by pathogens during handling, preparation, or storage. Contamination can occur through unwashed hands, dirty utensils, or contact with raw meat carrying bacteria such as Salmonella or E. coli.

Hygienic food preparation involves washing hands before handling food, thoroughly cooking food to kill pathogens, and preventing cross-contamination by using separate chopping boards for raw and cooked foods.

For example, cooking chicken to an internal temperature of at least 75°C ensures harmful bacteria are destroyed, preventing food poisoning outbreaks. Refrigeration slows bacterial growth, reducing the risk of foodborne illness.

• Pathogens contaminate food via poor handling or storage.
• Cooking food thoroughly kills harmful microorganisms.
• Preventing cross-contamination reduces infection risk.

Personal hygiene practices such as regular handwashing with soap, bathing, and oral hygiene reduce the number of pathogens on the body and hands, which are common transmission routes for diseases.

Hands are a major vector for transferring pathogens to the mouth, eyes, or other people. Washing hands after using the toilet and before eating removes pathogens like viruses and bacteria.

For instance, handwashing with soap can reduce diarrhoeal disease incidence by up to 40%. Good hygiene also limits the spread of respiratory infections by reducing pathogen transfer via touch.

• Hands transfer pathogens between people and surfaces.
• Regular handwashing removes infectious agents.
• Good hygiene lowers transmission of many diseases.

Improper disposal of solid waste and rubbish can create breeding grounds for disease vectors such as flies, rats, and mosquitoes, which carry pathogens to humans.

Disposing of waste in designated bins and landfills prevents environmental contamination and reduces contact between humans and harmful organisms.

For example, stagnant water in discarded containers can breed mosquitoes that transmit malaria and dengue fever. Proper waste management interrupts these transmission cycles.

• Waste attracts disease vectors like flies and mosquitoes.
• Proper disposal reduces environmental contamination.
• Interrupts vector-borne disease transmission.

Sewage contains human waste that carries pathogens including bacteria, viruses, and parasites. If untreated, sewage contaminates water bodies used for drinking or recreation, spreading diseases.

Sewage treatment processes remove or kill pathogens before releasing water back into the environment, protecting water quality and public health.

Although detailed stages are not required, treatment typically involves physical removal of solids, biological breakdown of organic matter, and disinfection to eliminate pathogens. This prevents outbreaks of diseases such as cholera and hepatitis A.

• Sewage contains high levels of pathogens.
• Treatment removes pathogens before water release.
• Protects water sources and reduces disease risk.

Active immunity is a defence mechanism where the body produces its own antibodies in response to the presence of a pathogen. When a pathogen enters the body, the immune system recognises molecules on the pathogen called antigens as foreign. This triggers a complex immune response that leads to the production of antibodies specifically designed to bind to those antigens and neutralise or destroy the pathogen.

Unlike passive immunity, where antibodies are received from another source, active immunity involves the body's own immune cells learning to recognise and remember the pathogen, enabling a faster and stronger response if the pathogen invades again.

• Active immunity requires exposure to a pathogen or antigen.
• It involves antibody production by the body's immune system.
• It leads to immunological memory for long-term protection.

When a pathogen invades, specialised white blood cells called lymphocytes detect its antigens. B lymphocytes (B cells) respond by producing antibodies that specifically match the pathogen's antigens. These antibodies bind to the pathogen, marking it for destruction or neutralising its harmful effects.

The process includes clonal selection, where only B cells that recognise the antigen multiply rapidly to produce large amounts of antibodies. Some of these B cells become memory cells, which remain in the body for years, ready to respond quickly if the same pathogen appears again.

For example, if a person is infected by the influenza virus, their B cells produce antibodies that target the virus's surface proteins, helping to clear the infection.

• Lymphocytes recognise pathogen antigens.
• B cells produce specific antibodies.
• Memory cells provide long-term immunity.

Active immunity can be acquired naturally or artificially. Natural active immunity occurs when a person is infected by a pathogen and recovers, having produced antibodies during the infection. Artificial active immunity is induced by vaccination, where a harmless form or part of the pathogen (antigen) is introduced to stimulate antibody production without causing disease.

For example, recovering from chickenpox gives natural active immunity, while receiving the measles vaccine provides artificial active immunity. Both methods result in the immune system producing antibodies and memory cells.

This distinction is important because vaccination allows immunity to develop safely without the risks associated with natural infection.

• Natural active immunity follows infection and recovery.
• Artificial active immunity results from vaccination.
• Both involve antibody production and memory cell formation.

Vaccination introduces antigens from a pathogen into the body in a controlled way, such as weakened or dead pathogens, or isolated proteins. This stimulates the immune system to produce antibodies and memory cells without causing the disease.

For example, the tetanus vaccine contains inactivated toxin from the bacterium Clostridium tetani, prompting antibody production that protects against future infection.

Vaccination programs help prevent outbreaks by creating herd immunity, where enough people are immune to reduce disease spread.

• Vaccines contain harmless antigens to stimulate immunity.
• They induce antibody production and memory cells.
• Vaccination provides safe, long-term protection.

Active immunity provides long-term protection because memory cells remain in the body for years or even a lifetime. If the same pathogen invades again, these memory cells quickly produce large amounts of specific antibodies, preventing illness or reducing severity.

Antibody specificity means antibodies bind only to particular antigens on a pathogen. This ensures the immune response targets the correct invader without harming the body's own cells.

For example, after recovering from measles, memory cells enable rapid antibody production if exposed again, usually preventing reinfection.

• Memory cells enable rapid response on re-exposure.
• Antibodies are specific to pathogen antigens.
• Active immunity can last for many years.

Active immunity is a defence mechanism where the body produces its own antibodies in response to a pathogen. This immunity is specific to the pathogen encountered and usually provides long-term protection. Unlike passive immunity, where antibodies are received from another source, active immunity requires the immune system to be stimulated to respond and remember the pathogen.

• Active immunity involves antibody production by the individual's immune system.
• It provides long-lasting protection due to immune memory.
• It can be acquired naturally or artificially.

When a pathogen enters the body and causes an infection, the immune system recognises foreign molecules called antigens on the pathogen's surface. White blood cells called lymphocytes respond by producing antibodies specific to these antigens. This antibody production helps to neutralise or destroy the pathogen. After the infection is cleared, some lymphocytes become memory cells, which remain in the body and enable a faster and stronger response if the same pathogen invades again.

• Pathogen infection triggers antibody production.
• Antibodies target specific antigens on the pathogen.
• Memory cells form to provide future immunity.
• Example: Recovering from chickenpox usually leads to lifelong immunity.

Vaccination introduces a harmless form of a pathogen or its antigens into the body. This can be a weakened (attenuated) pathogen, a dead pathogen, or isolated antigen molecules. The immune system recognises these antigens and produces antibodies and memory cells without causing the disease symptoms. This prepares the immune system to respond rapidly and effectively if exposed to the live pathogen later, preventing illness.

• Vaccines contain harmless forms or parts of pathogens.
• They stimulate antibody and memory cell production.
• Vaccination provides immunity without causing disease.
• Example: The measles vaccine protects against measles infection.

Antibodies are proteins that specifically bind to antigens on pathogens, marking them for destruction or neutralising their harmful effects. Memory cells are long-lived lymphocytes that remember the specific antigen. On subsequent exposures to the same pathogen, memory cells enable the immune system to produce antibodies more quickly and in greater quantities, often preventing reinfection or reducing disease severity.

• Antibodies bind to and neutralise pathogens.
• Memory cells enable rapid secondary immune response.
• This leads to long-lasting immunity after infection or vaccination.

Active immunity is essential for long-term protection against infectious diseases. Natural infection can provide immunity but often at the risk of illness and complications. Vaccination offers a safer way to develop active immunity by stimulating the immune system without causing disease. Understanding active immunity helps explain how vaccines protect populations and why booster doses may be needed to maintain immunity.

• Active immunity provides durable protection.
• Natural infection carries risks but induces immunity.
• Vaccination safely induces active immunity.
• Booster vaccines may be required to maintain immunity.