Sec 4 Combined Science (Biology) Infectious Diseases notes

Infectious diseases are caused by pathogens and can pass from one person to another: influenza, pneumococcal disease, food poisoning, the common cold. Non-infectious diseases develop inside the body and cannot be caught: type 2 diabetes (met in Chapter 4), most cancers, heart disease linked to lifestyle. The dividing test is simple: *can a healthy person catch it from an infected one?* If yes, a pathogen is hiding somewhere in the story.

A pathogen is any organism that causes disease. The two groups in this course are bacteria — tiny single cells that damage tissue or release toxins — and viruses, far smaller, which cannot reproduce except inside the cells of the organism they infect. Bacteria cause pneumococcal disease; viruses cause influenza and the common cold. The distinction decides treatment later: antibiotics work on bacteria only.

Pathogens ride three main routes. Body fluids: droplets flung out by coughs and sneezes, saliva, mucus, and blood. Contaminated food: undercooked or badly stored food carrying microbes. Contaminated water: water tainted with sewage or pathogens, a major route for gut diseases. Block all three routes — wash hands, cook food properly, treat drinking water — and transmission collapses.

Influenza (flu) is a viral infection. Its signs and symptoms: sudden fever, sore throat, cough, headache, muscle aches and fatigue. It spreads chiefly in droplets from coughs and sneezes, and by touching contaminated surfaces then the face. Pneumococcal disease is caused by the bacterium Streptococcus pneumoniae. It most often appears as pneumonia — a lung infection with high fever, chills, chest pain, difficulty breathing and cough — but the same bacterium can invade the blood or the coverings of the brain, causing meningitis with stiff neck and confusion. It spreads in droplets and by direct contact with mucus or saliva.

A vaccine contains an agent that resembles a pathogen — a harmless form or fragment of it. The body responds as if infected: white blood cells are stimulated to produce antibodies against the look-alike. Because the vaccine cannot cause the disease, nothing harmful happens — but the immune system has been drilled. On real infection the white blood cells recognise the pathogen and produce antibodies quickly, destroying it before it can multiply enough to cause illness: the person is protected, or immune.

This is why vaccination is a population-level defence too: flu vaccines before the season, pneumococcal vaccines for the vulnerable. Both of the named diseases have vaccines — which, combined with hygiene, is how their spread is reduced.

The mark-scheme version to memorise: *vaccines contain an agent resembling a pathogen → white blood cells stimulated → antibodies produced → on real infection antibodies are produced quickly → disease prevented.*

Antibiotics are drugs that kill bacteria — they interfere with bacterial cells, which have structures our cells lack. They are ineffective against viruses, because viruses do not have those bacterial structures; they hide inside our own cells, where an antibiotic cannot reach them without harming us. Taking antibiotics for flu does nothing against the virus — it only exposes the body's harmless bacteria to selection.

That exposure is how antibiotic resistance accelerates. In any bacterial population, a few cells happen to be less affected by the drug. If the antibiotic is used wrongly — skipped doses, half-finished courses, unnecessary prescriptions — the susceptible bacteria die but the resistant ones survive and multiply, and soon the drug works poorly or not at all. Every careless course of antibiotics is a training camp for tougher bacteria.