Sec 4 Biology Infectious Diseases notes

A disease is infectious if a pathogen — a disease-causing organism — causes it and it can pass from one host to the next. Diseases like type 2 diabetes or coronary heart disease have no pathogen and cannot be caught: they are non-infectious. The syllabus concentrates on two pathogen kinds.

A virus is minimal: a core of genetic material (DNA or RNA) inside a protein coat. It has no cytoplasm, no organelles and no metabolism of its own, so it can reproduce only inside a living host cell — hijacking the cell’s machinery to copy itself, then bursting out to infect new cells. A bacterium, by contrast, is a complete living cell: a cell wall around a membrane, cytoplasm, and DNA loose in the cell with no nucleus around it. Crucially, most bacteria are harmless or useful (gut flora, decomposers); only some species are pathogenic.

Pathogens ride the routes people share. Body fluids — blood, saliva, sexual fluids — carry viruses and bacteria between hosts during contact, shared needles or unprotected sex. Contaminated food and water deliver gut pathogens when hygiene fails: unwashed produce, undercooked meat, sewage-touched supplies. And respiratory infections travel in droplets sprayed by coughs and sneezes, hanging in the air of crowded, poorly ventilated rooms.

Each route has a matching brake: clean drinking water and proper sewage treatment, thorough cooking and food hygiene, handwashing, covering coughs, ventilation, and avoiding contact with other people’s blood or body fluids. Public health is mostly breaking transmission routes.

Influenza (flu) is a viral infection of the air passages. Its signs and symptoms include fever, headache, sore throat, coughing, muscle aches and tiredness. It spreads in droplets from coughs and sneezes and via contaminated hands and surfaces. Reduction: annual vaccination (the virus changes, so top-ups are needed), covering the mouth and nose when coughing, handwashing, ventilation of crowded rooms, and staying home while infectious.

Pneumococcal disease is caused by the bacterium Streptococcus pneumoniae. It ranges from ear and sinus infections to pneumonia (lung infection) and even meningitis (infection of the membranes around the brain). Symptoms follow the site: fever, cough and chest pain in pneumonia; fever, severe headache and a stiff neck in meningitis. It likewise spreads in droplets and by close contact. Reduction: the pneumococcal vaccine, plus the same hygiene measures — handwashing, covering coughs, avoiding overcrowding.

A vaccine contains an agent that resembles a pathogen — a killed or weakened form, or fragments of it. The vaccine is harmless, but the immune system takes it seriously: white blood cells respond by producing antibodies specific to that pathogen and, crucially, by keeping memory cells. If the real pathogen arrives later, the memory cells trigger a faster, stronger secondary response that destroys it before disease develops — that is immunity.

Vaccination therefore protects without the patient ever suffering the illness — and when most of a community is vaccinated, pathogens meet so few susceptible hosts that spread stalls, shielding even the unvaccinated.

Antibiotics are drugs that kill bacteria or stop them growing — typically by attacking the bacterial cell wall or essential bacterial machinery. Viruses have none of those targets: no cell wall, no cytoplasm, no own metabolism. That is why antibiotics cannot treat viral infections such as influenza — taking them for flu helps nothing and fuels resistance.

Within any bacterial population, chance mutations make a few cells resistant. If the antibiotic is used correctly, the susceptible majority dies and the immune system clears the leftovers. But misuse and overuse — unnecessary courses, half-finished courses, low doses — do the opposite: they kill the susceptible bacteria and leave the resistant ones to multiply without competition. The result is an infection that the same drug can no longer cure, and resistant strains that spread between people. The defences: take antibiotics only when prescribed, complete the full course, and never share or reuse leftovers.