Everything from the 3 lessons of this topic on a few pages: the key idea of each lesson, the diagram to remember, the points to learn and the traps examiners set. Read it, cover it, test yourself.
An infectious disease is caused by a pathogen, an organism or virus that causes disease, and it is transmissible: it can be passed from one host to another. Cholera is caused by the bacterium Vibrio cholerae and spreads by the faecal-oral route in contaminated water and food. Malaria is caused by the protoctists Plasmodium falciparum, P. malariae, P. ovale and P. vivax and is carried by a vector, the female Anopheles mosquito. Tuberculosis (TB) is caused by the bacteria Mycobacterium tuberculosis and M. bovis and spreads in airborne droplets, with M. bovis also caught from infected cattle in unpasteurised milk or undercooked meat. HIV/AIDS is caused by the human immunodeficiency virus and is passed in body fluids: unprotected sex, shared needles and infected blood, and from mother to baby.
Malaria is not caused by a bacterium or by the mosquito. The pathogen is the protoctist Plasmodium; the mosquito is only the vector that carries it. Name the pathogen with its genus (and species where asked), written in italics or underlined.
HIV does not survive long outside the body and is not spread by touching, coughing, sharing cups or by mosquitoes. A mosquito transmits Plasmodium because the parasite completes part of its life cycle inside the insect; HIV does not.
Control measures break the chain of transmission at one of three points: treating infected people so they stop passing the pathogen on, blocking the route (clean water, mosquito nets, condoms) or protecting the next host (vaccination). Each one is limited by biological factors (drug and insecticide resistance, pathogens hiding inside cells, many strains or changing antigens, symptomless carriers, no vaccine for HIV), social factors (crowding in refugee camps and cities, natural disasters and war, stigma, whether people use nets or condoms or finish treatment) and economic factors (the cost of sewage systems, drugs, nets and long courses of treatment, often in the poorest countries). Details of the malaria life cycle are not needed.
A treatment that saves lives does not always reduce transmission. Oral rehydration solution prevents death from dehydration in cholera, but the patient still releases bacteria in faeces. In the model the curves do not change when only that box is ticked.
Do not just list measures. A discuss question needs the factor and its effect: for example, mosquitoes have become resistant to insecticides so spraying kills fewer of them and the vector survives to transmit Plasmodium.
An antibiotic is a chemical that kills or stops the growth of bacteria without harming human cells. Penicillin acts on growing bacteria: it inhibits the transpeptidase enzymes that form cross-links between the peptidoglycan chains of new cell wall. The wall becomes weak, water enters the cell by osmosis, and the cell swells and bursts (osmotic lysis). Human cells have no cell wall or peptidoglycan, so they are not affected. Antibiotics do not affect viruses: viruses have no cell wall, no peptidoglycan, no ribosomes and no metabolism of their own, and they replicate inside host cells using the host's enzymes. Antibiotic resistance arises by chance mutation; each use of an antibiotic selects resistant bacteria, which multiply and pass on the resistance gene. The consequences are infections such as MRSA and MDR-TB that are hard or impossible to treat. The impact is reduced by prescribing antibiotics only when needed, finishing the course, hospital hygiene, less use in farming, changing the antibiotic used and developing new ones.
Penicillin does not dissolve or digest the wall that is already there. It stops new cross-links forming, so it works only on bacteria that are growing. The cell is killed by water entering by osmosis, not by penicillin entering the cytoplasm.
Bacteria do not become resistant because they "get used to" an antibiotic, and people do not become resistant: bacteria carry resistance genes that arose by chance mutation, and the antibiotic selects them.