Answer: Influenza (flu) → Virus; Tuberculosis → Bacterium; Athlete's foot (tinea) → Fungus; Malaria → Protist

Answer: They can only reproduce inside the cells of another organism

Viruses are not cells. They take over a host cell and use it to make copies of themselves.

Answer: Any two, for example: droplets in the air from coughs and sneezes (cover coughs, wear masks, stay home when sick); direct contact or touching contaminated surfaces (wash hands); contaminated food or water (cook food properly, treat drinking water); animal vectors such as mosquitoes (use insect repellent and nets).

Answer: 64

1. 2 hours = 120 minutes
2. 120 ÷ 20 = 6 doublings
3. 1 × 2 × 2 × 2 × 2 × 2 × 2 = 2⁶ = 64

Answer: 6400

1. 3 hours = 180 minutes; 180 ÷ 30 = 6 doublings
2. 100 × 2⁶ = 100 × 64 = 6400

Answer: Acid in the stomach

The first line stops pathogens from getting in: skin, mucus, tiny hairs (cilia), tears and stomach acid.

Answer: They are white blood cells that engulf (surround and take in) pathogens and digest them.

Answer: A higher body temperature slows the growth and reproduction of many pathogens and can speed up the action of the immune system.

Answer: False

Antibodies are specific: each type fits only one antigen, like a lock and key, so a different antibody is needed for each pathogen.

Answer: After the first infection, memory cells remain in the blood. If the same virus enters again, they recognise it and make the right antibodies very quickly, destroying the virus before you become ill. You are immune.

Answer: It contains dead or weakened pathogens, or harmless parts of them (such as their antigens), or instructions for making those antigens. These cannot cause the disease, but they still trigger the immune system to make antibodies and memory cells.

Answer: 90%

1. 450 ÷ 500 × 100 = 90%

Answer: When most people are immune, a pathogen cannot easily find new hosts to infect, so it cannot spread through the community. This means unvaccinated people are much less likely to come into contact with it.

Answer: Antibiotics kill bacteria but have no effect on viruses

Antibiotics act on structures and processes in bacterial cells. Using them when they are not needed also helps antibiotic-resistant bacteria to develop.

Model answer: On first exposure, the lymphocytes that make the right antibody must first recognise the new antigen and multiply, which takes several days, so antibody levels rise slowly and the person may become ill. During this response, memory cells are produced, which stay in the body for a long time. On a second exposure to the same pathogen, the memory cells recognise its antigens straight away and quickly make large amounts of antibody, destroying the pathogen before it can cause illness. Vaccination uses this by giving the first exposure safely: the vaccine's antigens cause memory cells to form without disease, so when the real pathogen arrives, the fast secondary response protects the person.