In the 1840s, at a Vienna hospital, Ignaz Semmelweis noticed something terrible. On the ward run by doctors, far more mothers died of fever after giving birth than on the ward run by midwives. The doctors often came straight from dissecting bodies. Semmelweis made them wash their hands in a chlorine solution, and deaths fell sharply. Many doctors refused to believe him: nobody yet knew that invisible microorganisms cause disease. That idea, the germ theory, came later with Louis Pasteur and Robert Koch. Topic 3 is built on it.
Pathogens
Four kinds of microorganism cause communicable disease:
| Pathogen | Example | How it spreads |
|---|---|---|
| Virus | Measles, HIV, tobacco mosaic virus | Droplets; body fluids; contact between plants |
| Bacterium | Salmonella, gonorrhoea | Food; sexual contact |
| Protist | Malaria | Mosquitoes (the vector) |
| Fungus | Rose black spot | Spores in water and wind |
Bacteria may release toxins; viruses reproduce inside your cells, damaging them.
Your defences
First, keep pathogens out: skin, the nose, mucus and cilia in the airways, and stomach acid. If they get in, white blood cells engulf them (phagocytosis), make antibodies that target them, and make antitoxins.
Vaccines
In 1796 Edward Jenner noticed that milkmaids who'd had mild cowpox seemed safe from smallpox. He tested the idea on a boy, and it worked. A vaccine contains dead or inactive pathogen, so your white blood cells learn to make the right antibodies and respond fast next time. When most people are vaccinated, a disease struggles to spread: herd immunity.
Medicines
Antibiotics kill bacteria inside the body but don't work on viruses. Alexander Fleming noticed in 1928 that a mould stopped bacteria growing; Howard Florey and Ernst Chain spent years turning it into the medicine penicillin. Overusing antibiotics helps resistant bacteria spread.
New drugs are tested for toxicity, efficacy and dose: first on cells, tissues and animals, then in clinical trials on healthy volunteers and patients. In a double-blind trial, neither patients nor doctors know who gets the placebo.
Common exam mistakes
- "Antibiotics kill viruses." They only kill bacteria.
- "Mosquitoes cause malaria." A protist does; mosquitoes carry it.
- "Vaccines contain antibodies." They contain dead or inactive pathogen; your body makes the antibodies.
- Mixing up single-blind and double-blind. Double-blind: nobody knows until the end.
Try it
Say each answer before you flip. Then imagine you're Semmelweis, sure that something on the doctors' hands is killing patients but unable to see it. What would you have tested next?
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