Communicable Diseases

A pathogen is a microorganism that causes an infectious (communicable) disease. The four groups are bacteria, viruses, fungi and protists.

Bacteria reproduce rapidly inside the body and release toxins; these toxins damage cells and tissues, producing symptoms. Viruses must enter living host cells and use the cell's machinery to copy themselves; new virus particles burst out, killing the host cell. The accumulating cell damage produces the symptoms of viral disease.

(1) Treat or boil drinking water to kill bacteria. (2) Provide soap and hand-washing facilities to break the faecal–oral route. (3) Isolate infected individuals so they do not contaminate shared water supplies.

Children are vaccinated with the MMR vaccine (measles, mumps, rubella). Infected people stay off school/work to prevent droplet spread. High vaccination rates produce herd immunity, protecting babies and immunocompromised people who can't be vaccinated.

The female Anopheles mosquito is the vector. (1) When the mosquito bites an infected human, it takes up Plasmodium protists in the blood. (2) The protists develop and multiply in the mosquito's gut. (3) When the mosquito bites a new human, protists in its saliva are injected into the bloodstream. (4) Without the mosquito, the protist cannot pass from person to person.

The mosaic patches contain less chlorophyll, so they absorb less light energy for photosynthesis. With reduced photosynthesis, the plant produces less glucose, so it grows more slowly and produces less fruit/crop — reducing yield.

(1) Skin forms a tough barrier of dead cells. (2) Hair and mucus in the nose trap microbes from inhaled air. (3) Cilia in the trachea/bronchi beat mucus (with trapped microbes) up to the throat. (4) Hydrochloric acid in the stomach kills swallowed pathogens.

A phagocyte recognises and binds to the pathogen. It engulfs the pathogen by wrapping its cell membrane around it, taking it inside a vesicle. Enzymes inside the phagocyte digest the pathogen into harmless products.

Antibodies are specific — each antibody has binding sites shaped to fit one particular antigen, like a key in a lock. Measles viruses and flu viruses have different antigens on their surfaces, so the measles antibody's binding sites don't fit flu antigens. The body needs a different antibody for each pathogen.

The MMR vaccine contains a small amount of dead or inactive measles virus. Lymphocytes recognise the antigens on the virus and produce specific antibodies. Some lymphocytes become memory cells that remain in the blood. If the child later meets live measles virus, the memory cells make antibodies very quickly and in large amounts, destroying the virus before symptoms develop.

On first exposure, lymphocytes have to find the matching antibody type and divide, so the response is slow and small. After this, memory cells remain in the blood. On second exposure, memory cells are already specific to the antigen, so they make large amounts of antibody very quickly — giving a much higher and faster peak.

Measles is highly infectious, so it needs roughly 95 % of the population to be immune to prevent spread. Dropping to 80 % means too many people are still susceptible — the virus can spread from person to person and cause outbreaks. People who cannot be vaccinated (such as newborn babies or those on chemotherapy) lose herd immunity protection and are most at risk.

Vaccines trigger the production of memory cells so that future infections are destroyed before symptoms appear. Advantages: vaccinated children are protected from serious illnesses such as measles and whooping cough; high coverage (>90 %) gives herd immunity that protects babies too young to be vaccinated. Disadvantages: vaccines are not 100 % effective — some children still catch the disease, though usually mildly; minor side effects (sore arm, brief fever) and very rare serious reactions can occur. On balance, the very large public-health benefits and NHS cost savings outweigh the small risks, which is why the UK schedule is recommended for all children.

Antibiotics destroy bacterial cells (for example by disrupting their cell walls) so they cure the bacterial chest infection. Viruses are not cells — they live and reproduce inside the body's own cells. A drug that killed the virus would also damage the body cell, so antibiotics are useless for flu and would not be prescribed.

Paracetamol reduces the patient's fever and pain — it treats the symptoms of the infection. It does NOT kill the virus; the body's immune system (white blood cells producing antibodies) clears the virus.

Random mutations in the bacterial DNA can produce a bacterium that is resistant to the antibiotic. When the antibiotic is used, non-resistant bacteria are killed but the resistant bacterium survives. It reproduces rapidly, passing on the resistance gene to its offspring. Over many generations, the proportion of resistant bacteria in the population rises until the antibiotic is no longer effective — this is natural selection.

When symptoms ease, some bacteria are still alive in the body. The bacteria still surviving are likely to be the most resistant to the antibiotic. If treatment stops early, these survivors reproduce and a new, more resistant strain develops. Finishing the full course ensures all the bacteria are killed and reduces the chance of resistance spreading.

(1) Digitalis is extracted from the foxglove plant and is used to treat heart conditions. (2) Aspirin was originally extracted from willow bark and is used as a painkiller and to reduce inflammation. (3) Penicillin was discovered by Alexander Fleming in 1928 in the Penicillium mould and is used as an antibiotic to kill bacterial infections.

Toxicity — is the drug safe (does it harm cells or tissues)? Efficacy — does it work (does it treat the disease)? Dose — what amount gives the best effect with the fewest side effects?

The drug is first tested in the laboratory on cells and tissues to check it has an effect and isn't toxic. It is then tested on live animals to check effects on whole-body systems. Clinical trials follow: in Phase 1 the drug is given in very low doses to healthy volunteers to check for safety and side effects. In Phase 2 it is given to patients with the disease, to test efficacy and refine the dose. In Phase 3 it is given to a large number of patients in a double-blind placebo-controlled trial to confirm efficacy and detect rare side effects. Finally the data are reviewed by the UK MHRA, which decides whether to approve the drug for use on the NHS.

A placebo is a dummy treatment that looks identical to the real drug but contains no active ingredient. Some patients receive the placebo so their outcomes can be compared with those who get the real drug — this controls for the placebo effect, where patients feel better simply because they expect to. In a double-blind trial, neither the patient nor the doctor knows who is receiving the drug and who the placebo. This prevents both patient and doctor bias from influencing the results.