Global challenges Study Guide

Infections and diseases are conditions that impair the normal functioning of organisms. This explanation focuses on three key areas: a common human infection (influenza), a plant disease (tobacco mosaic virus), and sexually transmitted infections (STIs) in humans, specifically HIV/AIDS.

Influenza, commonly known as the flu, is a highly contagious respiratory illness caused by influenza viruses. It spreads primarily through airborne droplets released when infected individuals cough or sneeze. Symptoms typically include fever, cough, sore throat, muscle aches, and fatigue. The virus can mutate frequently, which is why annual vaccinations are often recommended. Prevention strategies include vaccination, good hand hygiene, and avoiding close contact with sick individuals. Treatment often involves rest, fluids, and antiviral medications in some cases.

Tobacco Mosaic Virus (TMV) is a widespread plant pathogen that affects a variety of plant species, particularly tobacco and other members of the Solanaceae family (e.g., tomatoes, peppers). The virus causes characteristic mosaic patterns of light and dark green on leaves, stunting of growth, and reduced yield. TMV can be transmitted through contact between infected and healthy plants, through contaminated tools, or via human activity. It is very stable and can survive in dried plant material for many years. There is no cure for TMV once a plant is infected, so prevention is key. This involves sanitation, removing infected plants, and using resistant varieties.

Sexually Transmitted Infections (STIs) are infections passed from one person to another through sexual contact. HIV (Human Immunodeficiency Virus) is an STI that attacks the body's immune system. Over time, if left untreated, HIV can lead to AIDS (Acquired Immunodeficiency Syndrome), which is the most advanced stage of HIV infection. HIV is primarily spread through specific body fluids, including blood, semen, pre-seminal fluid, rectal fluids, vaginal fluids, and breast milk. Transmission typically occurs through unprotected sexual contact, sharing needles, or from mother to child during pregnancy, childbirth, or breastfeeding. Symptoms of initial HIV infection can include flu-like symptoms, which may then subside for many years. AIDS is diagnosed when a person with HIV develops certain opportunistic infections or cancers, or has a very low count of specific immune cells (CD4 cells). Prevention of HIV/AIDS mainly involves safe sexual practices (e.g., condom use), not sharing needles, and antiretroviral therapy (ART) for infected individuals, which reduces the viral load and the risk of transmission. ART also helps people with HIV live longer, healthier lives.

The human immune system is a complex network of cells, tissues, and organs that work together to protect the body from pathogens, which are disease-causing microorganisms like bacteria, viruses, fungi, and parasites. This defence mechanism can be broadly categorised into two main types: non-specific defences (also known as innate immunity) and specific defences (also known as adaptive immunity). Non-specific defences provide a rapid, general response to any pathogen. These include physical barriers such as skin, which prevents pathogens from entering the body, and chemical barriers like stomach acid, which kills ingested pathogens, and mucus, which traps pathogens. Additionally, phagocytes, a type of white blood cell, engulf and destroy pathogens in a non-specific manner. If pathogens breach these initial defences, the specific immune system is activated. This system involves specialised white blood cells called lymphocytes, primarily B-lymphocytes and T-lymphocytes. B-lymphocytes produce antibodies, which are proteins that specifically target and neutralise particular pathogens or their toxins. T-lymphocytes can directly kill infected body cells or help activate other immune cells. A key feature of the specific immune system is its ability to develop immunological memory. After initial exposure to a pathogen, the body retains memory cells that can quickly recognise and mount a much faster and stronger response if the same pathogen is encountered again. This is the principle behind vaccination, where individuals are exposed to a weakened or inactivated form of a pathogen to stimulate memory cell production without causing illness, thereby providing future immunity. The coordinated action of both non-specific and specific defences ensures the body is well-protected against a wide range of diseases.