Infection & response flashcards

Key points

• A pathogen is a microorganism that causes infectious disease.
• The four types are bacteria, viruses, fungi and protists.
• Diseases caused by pathogens are communicable: they can pass from one organism to another.
• Pathogens infect plants as well as animals.

Key points

• Both reproduce rapidly inside the body.
• Bacteria release toxins, poisons that damage tissues and cause symptoms such as fever.
• Viruses live and reproduce inside your cells, and the cells are damaged as new viruses are released.
• A virus is not a cell: it can only copy itself inside a host cell.

Key points

• Viruses: measles (fever and a red rash), HIV (flu-like at first, and can lead to AIDS) and tobacco mosaic virus in plants.
• Bacteria: salmonella (food poisoning, often from undercooked chicken) and gonorrhoea (a sexually transmitted disease).
• Fungus: rose black spot, purple or black spots on leaves, which drop early, so the plant photosynthesises less.
• Protist: malaria, carried by mosquitoes, causing repeated bouts of fever.

Key points

• By air, in droplets from coughs and sneezes: measles.
• By water or food: salmonella in undercooked food.
• By direct contact, including sexual contact or sharing body fluids: HIV and gonorrhoea.
• By a vector, an organism that carries the pathogen: mosquitoes carry the malaria protist.
• Block the route: hygiene, isolating infected people, destroying or controlling vectors, and vaccination.

Key points

• Skin is a barrier and releases chemicals that kill microbes. Platelets clot the blood to seal a cut.
• In the nose, hairs and mucus trap particles that may carry pathogens.
• In the trachea and bronchi, mucus traps pathogens and cilia move it up to the throat to be swallowed.
• The stomach makes hydrochloric acid, which kills most pathogens that are swallowed.

Key points

• Phagocytosis: the white blood cell engulfs the pathogen and digests it.
• Antibodies: proteins that bind to the specific antigens on a pathogen, helping to destroy it.
• Antitoxins: proteins that neutralise the toxins some bacteria release.
• Each antibody fits only one antigen, so a new pathogen needs new antibodies, which takes time.

Key points

• The first time, white blood cells take time to make the right antibodies, so you may become ill.
• Afterwards, the white blood cells remember the pathogen.
• If it gets in again, they make the right antibodies quickly and in large numbers.
• The pathogen is destroyed before you feel ill: you are immune.

Key points

• A vaccine contains a small amount of a dead or inactive form of a pathogen.
• White blood cells respond by making the right antibodies, and they remember the pathogen.
• If the live pathogen gets in later, the antibodies are made fast, before you become ill.
• When most people are vaccinated, the pathogen cannot spread easily, which protects those who are not immune.

Key points

• Antibiotics, such as penicillin, kill bacteria inside the body.
• The right antibiotic has to be chosen for the bacteria causing the infection.
• Antibiotics have greatly reduced deaths from bacterial diseases.
• They do not work on viruses, which live inside the body's own cells, so are hard to kill without harming them.
• Painkillers, such as paracetamol, ease symptoms but kill no pathogens.

Key points

• A random mutation makes some bacteria resistant to an antibiotic.
• The antibiotic kills the other bacteria, but the resistant ones survive and reproduce.
• So the resistant strain spreads, and the antibiotic no longer treats it.
• Slow it: prescribe antibiotics only for serious bacterial infections, never for viruses, and finish the whole course.

Why resistance spreads so fast

Key points

• Many traditional drugs came from plants and microorganisms.
• Digitalis, a heart drug, comes from foxgloves; aspirin, a painkiller, was developed from willow bark.
• Alexander Fleming discovered penicillin, made by the Penicillium mould.
• Most new drugs are now made by chemists in the pharmaceutical industry, though the starting point may be a chemical from a plant.

Key points

• Every new drug is tested for toxicity (is it safe?), efficacy (does it work?) and dose (how much?).
• Preclinical testing: on cells, tissues and live animals.
• Clinical trials: first healthy volunteers at very low doses, to check it is safe.
• Then patients, to find the dose that works best.
• In a double-blind trial, neither patients nor doctors know who has the drug and who has a placebo.

Reading a trial's results