传染病

An infectious disease 传染病 is caused by a pathogen 病原体 — an organism that lives in or on a host and causes harm. Infectious diseases are transmissible: the pathogen can be transmitted 传播 (passed) from one person to another.

You need to know four diseases, the pathogen that causes each, and how each spreads.

HIV infects and destroys certain white blood cells, so it slowly weakens the body's immune system 免疫系统.

Control has biological, social and economic sides — the science of the pathogen, people's behaviour and education, and the money and resources available. Examples:

• cholera: provide clean water and proper sewage treatment; good hygiene; vaccines 疫苗 in some areas.
• malaria: sleep under nets; remove pools of still water where mosquitoes breed; spray insecticides 杀虫剂; take anti-malarial drugs.
• TB: find and treat infected people with a long course of antibiotics; give the BCG vaccine; reduce overcrowding; trace contacts of patients.
• HIV: use condoms; use clean needles; test donated blood; educate people. There is no cure and no vaccine yet, but drugs can slow the virus down.

In every case, cost (economic), people's willingness to change behaviour (social) and the supply of drugs or vaccines (biological) all affect how well a disease can be controlled.

An antibiotic 抗生素 is a drug that kills bacteria or stops them growing. For example, penicillin 青霉素 stops bacteria from building their cell walls 细胞壁. As the bacterium grows, its weak wall cannot hold it, so the cell takes in water and bursts.

Antibiotics do not work against viruses. A virus has no cell wall and no chemical reactions of its own to attack — it simply uses the machinery of the host cell. So there is no antibiotic target in a virus.

Sometimes a mutation 突变 makes a bacterium resistant to an antibiotic, which means the antibiotic no longer kills it. This resistance 耐药性 is a serious problem:

• when an antibiotic is used, the non-resistant bacteria die, but any resistant ones survive and multiply. Over time, more and more bacteria carry the resistance.
• some infections then become very hard, or impossible, to treat.

Steps to slow resistance down:

• only use antibiotics when they are really needed (not for viral illnesses such as colds).
• always finish the full course, so no bacteria are left alive.
• use the correct antibiotic for the infection.
• reduce the heavy use of antibiotics in farming.

Worked example. A patient with influenza is prescribed an antibiotic. Explain why it will not help, and why it is actively harmful. Antibiotics work by attacking structures or processes that bacteria have and human cells do not - penicillin, for instance, blocks cell wall synthesis, so the growing bacterium bursts under osmotic pressure. A virus has no cell wall, no ribosomes of its own and no metabolism: it replicates inside the host's cells using the host's machinery, so there is simply no bacterial target for the drug to attack. Taking it anyway exposes the patient's harmless resident bacteria to the antibiotic, killing the susceptible ones and selecting for resistant ones - which is how resistance spreads through a population. Name the target that is missing: "viruses are not alive" is not the reason and earns nothing.

Common mistakes.

• Calling the mosquito the pathogen of malaria. The pathogen is the protoctist Plasmodium; the mosquito is only the vector that carries it.
• Saying the bacteria become resistant because they meet the antibiotic. A resistant allele arises by chance mutation first; the antibiotic then selects it by killing the non-resistant bacteria.
• Saying people or the body become resistant to an antibiotic. It is the bacteria that are resistant.
• Saying HIV is the same as AIDS. HIV is the virus; AIDS is the disease that develops later, when so many white blood cells are destroyed that other infections take hold.
• Writing that penicillin "attacks the cell wall". It stops the bacterium building its cell wall as it grows, so the weakened cell takes in water by osmosis and bursts.
• Giving only biological control measures when a question asks about social or economic factors. Name the people side (education, crowded housing, behaviour) and the money side (cost of drugs, vaccines, clean water) as well.

• For each named disease give the pathogen and route (cholera – bacterium, water; malaria – Plasmodium, mosquito vector; also TB, HIV/AIDS).
• Explain why antibiotics work on bacteria only (they target cell walls/enzymes, not viruses).
• Explain antibiotic resistance as natural selection: a resistant mutant survives and reproduces; reduce it by finishing courses and avoiding overuse.