Principles of Pathogens, Immunity, and Disease Control

Pathogens and Transmissible Diseases\n\nA pathogen is a disease-causing organism, typically bacteria, fungi, protoctists, or viruses, that infects a host. All pathogens are microscopic and only visible via microscope. Transmissible diseases move between hosts via direct contact, such as blood and body fluids, or indirect contact through air, water, food, contaminated surfaces, or animal vectors like the mosquito carrying $Plasmodium$. Examples include cholera ($bacterium$), athlete's foot ($fungus$), malaria ($protoctist$), and the common cold ($virus$).\n\n# Human Defences and the Immune System\n\nThe first line of defence consists of mechanical barriers, including waterproof skin, blood-clotting scabs, and nose hairs, alongside chemical barriers such as sticky mucus and acidic stomach juices containing hydrochloric acid. Ciliated cells use cilia to move trapped particles away from the lungs. The second line involves white blood cells: neutrophils perform phagocytosis to engulf and destroy pathogens, while lymphocytes produce antibodies to mark or destroy them.\n\n# Hygiene and Disease Control\n\nSpread is controlled through safe water supplies, sewage treatment, and waste disposal in covered containers to deter vectors. Personal hygiene involves washing hands with soap and covering sneezes. Food hygiene requires refrigerating meat, fish, and dairy, using separate preparation tools to avoid $Salmonella$, and keeping food covered. Immunisation programs, which prevent over $2.5\,\text{million}$ deaths among children annually, have addressed diseases like smallpox, which previously killed up to $60\%$ of adults and over $80\%$ of infected children.\n\n# Active and Passive Immunity\n\nActive immunity is the long-term production of internal antibodies by lymphocytes in response to specific antigens. The primary response takes $7$ to $14\,\text{days}$, while memory cells enable a faster secondary response with up to $1000 \times$ more antibodies. Vaccination provides active immunity safely using dead or weakened pathogens. Passive immunity offers short-term, immediate protection using external antibodies from sources like the placenta, colostrum, or breast milk, but does not produce memory cells.\n\n# Cholera Pathogenesis\n\nCholera is caused by the bacterium $Vibrio cholerae$, which is found in contaminated water, a link confirmed by Dr John Snow in $1854$. Once inside the small intestine, the bacteria release a toxin that causes cells to secrete chloride ions into the lumen. This creates an osmotic shift that draws massive amounts of water into the intestine, leading to severe watery diarrhoea and dehydration.\n\n# Questions and Discussion\n\nIn response to how a pathogen is destroyed by a human neutrophil during a video demonstration, the process is through phagocytosis, where the cell surrounds, engulfs, and destroys the target. Interaction also emphasizes that antigens and antibodies are substances with specific, complementary shapes that allow them to fit together for an immune response.