Global challenges Study Guide

Communicable diseases, also known as infectious diseases, are caused by pathogens such as viruses, bacteria, protists, and fungi. These diseases can be spread between individuals in both animal and plant populations through various mechanisms. Understanding these pathways is crucial for controlling outbreaks and preventing widespread infections. For animals, common routes of transmission include direct contact (e.g., touching an infected individual or contaminated surfaces), indirect contact (e.g., through airborne droplets from coughing/sneezing, consuming contaminated food/water), and vectors (animals like mosquitoes or ticks that transmit pathogens). Some diseases can also be sexually transmitted. In plants, diseases spread through direct contact between infected and healthy plants, airborne spores (for fungi), insect vectors, contaminated soil, and even infected seeds or propagating material. The specific method of transmission depends on the type of pathogen and the host organism. For example, viruses tend to require a host cell to replicate and are often spread through direct contact, aerosols, or vectors. Bacteria can multiply independently and spread through food, water, airborne droplets, or direct contact. Fungi produce spores that can be carried by wind or water, while protists often rely on vectors or contaminated water sources. Effective control measures often target these specific transmission routes, such as vaccination to prevent person-to-person spread, sanitation to reduce waterborne diseases, or pesticides to control insect vectors.

White blood cells and platelets are crucial components of the blood, each with specific adaptations that enable them to perform their defence functions effectively. White blood cells, also known as leukocytes, are a diverse group but generally share key features that aid in pathogen combat. Phagocytes, a type of white blood cell, have a lobed nucleus and a cytoplasm rich in lysosomes containing digestive enzymes. Their amoeboid shape allows them to change form and squeeze through capillary walls (diapedesis) to reach infection sites. They engulf pathogens through phagocytosis, forming a phagosome, which then fuses with lysosomes to destroy the invading microbes. Lymphocytes, another type of white blood cell, have a large, round nucleus and less cytoplasm. They are adapted to recognise specific pathogens and produce antibodies or destroy infected cells, forming part of the body's specific immune response. Platelets, on the other hand, are not whole cells but small, anucleated fragments derived from megakaryocytes. Their small, irregular shape and sticky surface are key adaptations. When a blood vessel is damaged, these sticky surfaces allow them to adhere to the collagen fibres exposed at the injury site and to each other, forming a platelet plug. They also release clotting factors, which initiate the complex cascade leading to the conversion of fibrinogen to fibrin, forming a mesh that traps red blood cells and further seals the wound – preventing blood loss and blocking the entry of pathogens.