药物

A drug 药物 is any substance taken into the body that changes or affects the chemical reactions 化学反应 in the body. Many drugs are medicines that help to treat illness.

Antibiotics 抗生素 are drugs used to treat bacterial infections 感染. They kill bacteria 细菌, or stop them growing, but they do not affect viruses 病毒. This is why antibiotics cannot cure a cold or the flu, which are caused by viruses.

Why antibiotics do not affect viruses. An antibiotic works by damaging a part of the bacterial cell, for example its cell wall 细胞壁, or by stopping the bacterium's own reactions. A virus is not a cell. It has no cell wall and no reactions of its own; it can only reproduce inside our cells. So there is nothing in a virus for the antibiotic to attack.

To test which antibiotic works best, small paper discs soaked in different antibiotics are placed on a plate covered with bacteria. Where an antibiotic kills the bacteria, a clear ring with no growth appears around the disc. A bigger ring means the antibiotic is more effective.

Some bacteria are resistant 耐药 to an antibiotic — the antibiotic no longer kills them. These bacteria survive and multiply, so the antibiotic slowly becomes less effective for everyone.

(Supplement) To slow this down, antibiotics should be used only when they are really needed, and the full course should always be finished. Using antibiotics too often lets resistant bacteria, such as MRSA, spread.

Worked example. A patient stops taking their antibiotic as soon as they feel better, and the infection comes back worse. Explain using resistance. In the starting population a few bacteria are already resistant, by chance. The antibiotic kills the non-resistant ones, so those few survive, multiply, and come to make up most of the population - and the antibiotic no longer works on it. Stopping early leaves behind exactly the bacteria that are hardest to kill. Say the resistant bacteria already existed and survived: writing that the antibiotic "made them resistant" is wrong and loses every mark.

• "The patient becomes resistant to the antibiotic." It is the bacteria that are resistant, not the person.
• "The antibiotic makes bacteria resistant." The resistant bacteria were already there, by chance; the antibiotic kills the others, so the resistant ones are left to multiply.
• "Antibiotics kill all microorganisms." They kill bacteria only, not viruses.
• "A drug is an illegal substance." Medicines such as antibiotics and painkillers are drugs too.

• A drug is any substance that changes the body's chemical reactions — not only illegal drugs.
• Antibiotics kill bacteria only; they do not work on viruses (so they are useless against colds and flu).
• Overusing antibiotics leads to resistant bacteria like MRSA. Use them only when needed, and finish the whole course.