Communicable diseases caused by pathogens (bacteria, viruses, fungi and protists); examples in plants and animals; the human defence systems; vaccination, antibiotics and painkillers; and the discovery and development of drugs.

What this topic is asking

AQA wants you to describe the four types of pathogen and give disease examples, explain how the body defends itself, explain how vaccination and the immune system protect us, describe the roles of antibiotics and painkillers, and outline how new drugs are discovered and tested.

Pathogens and communicable disease

The AQA named examples are worth memorising precisely. Viral: measles (spread by droplets, can be fatal, prevented by vaccination), HIV (spread by body fluids, attacks immune cells and leads to AIDS if untreated) and tobacco mosaic virus in plants (causes a mosaic discolouration of leaves that reduces photosynthesis). Bacterial: salmonella food poisoning (causes fever, cramps and vomiting from the toxins it produces) and gonorrhoea (a sexually transmitted infection treated, where possible, with antibiotics). Fungal: rose black spot (purple or black spots on leaves, reducing growth, spread by water and wind). Protist: malaria (caused by a protist spread by the mosquito vector, causing recurrent fever). Pathogens spread by direct contact, contaminated water or food, the air (droplets), or vectors, so prevention focuses on hygiene, destroying vectors, isolation and vaccination.

Human defences and the immune system

The body's first line of defence stops pathogens entering and is both physical and chemical: the skin acts as a barrier and produces antimicrobial secretions, the nose has hairs and mucus that trap particles, the trachea and bronchi are lined with mucus and cilia that waft trapped pathogens away from the lungs, and the stomach produces hydrochloric acid that kills most pathogens in swallowed food and mucus.

After an infection, some white blood cells remain as memory cells, so if the same pathogen returns, the correct antibody is produced faster and in greater quantity, and the person is usually immune.

Vaccination, antibiotics and painkillers

Vaccination introduces small quantities of a dead or inactive form of a pathogen (or its antigens) so that white blood cells produce the matching antibodies and form memory cells, without the person becoming ill. A later infection by the live pathogen is then destroyed quickly. If a large proportion of the population is vaccinated, the pathogen cannot spread easily, which protects even those who are not vaccinated (herd immunity).

Antibiotics such as penicillin kill bacteria inside the body and have greatly reduced deaths from bacterial disease, but they do not kill viruses, because viruses live and reproduce inside the body's own cells. Overuse and misuse of antibiotics have led to antibiotic-resistant strains such as MRSA, which evolve by natural selection, so doctors now avoid prescribing antibiotics for non-serious or viral infections. Painkillers and other medicines treat the symptoms of disease but do not kill the pathogen.

Developing new drugs

Traditionally drugs were extracted from plants (digitalis from foxgloves, aspirin from willow) and microorganisms (penicillin from the Penicillium mould, discovered by Alexander Fleming). New drugs are first tested in preclinical trials on cells, tissues and then live animals, and only promising ones move to clinical trials on healthy volunteers (to check safety and find side effects) and then on patients, where the toxicity, efficacy and optimum dose are established. Trials often use a placebo and are double-blind, meaning neither the patient nor the doctor knows who has the real drug, which removes bias. Results are peer reviewed before publication to check the science is sound.