The ways pathogens are spread (direct contact, water, air and vectors), the named diseases for each route, and the methods used to reduce or prevent the spread of communicable diseases.

What this dot point is asking

AQA wants you to describe how pathogens are spread, link each route to a named disease, and explain practical methods of reducing or preventing the spread of communicable diseases in humans and plants.

How pathogens spread

• Direct contact: touching infected people, surfaces or plants. Examples: gonorrhoea (spread by sexual contact) and rose black spot (spread by touching infected leaves).
• Water: drinking or contact with water carrying pathogens, which is common where sanitation is poor, for example cholera.
• Air: pathogens carried in tiny droplets released when an infected person coughs or sneezes, which other people breathe in. Example: measles.
• Vectors: animals that carry a pathogen from one host to another without getting ill themselves. Example: mosquitoes spread the malarial protist; insects spread some plant diseases.

Reducing the spread of disease

Methods of reducing or preventing the spread, in humans, include:

• Hygiene: hand washing, using disinfectants, and keeping food preparation clean to avoid food poisoning.
• Isolation: keeping infected people away from others (quarantine) to stop transmission.
• Destroying vectors: for example draining standing water where mosquitoes breed, using insecticides, or using mosquito nets to stop bites.
• Vaccination: immunising people so the pathogen cannot spread easily through a population, which protects even those who are not vaccinated (herd immunity).

These methods each target a step in the chain of transmission, so AQA likes you to link the method to the route. For example, isolation works for an airborne disease because it removes the source of the droplets, hand washing works for a contact disease because it removes pathogens from the hands, and providing clean water works for a water-borne disease because it removes the pathogen from what people drink.

Vaccination deserves special mention because it works at the level of the whole population. If a large enough proportion of people are immune, the pathogen cannot find enough susceptible hosts to spread between, so even unvaccinated people are protected. This herd immunity is why governments run vaccination programmes against diseases such as measles, and why a fall in vaccination rates can allow a disease to return. AQA may give you data on vaccination rates and disease cases and ask you to explain the link, so understanding why widespread vaccination reduces spread is important.

Try this

Q1. Name one disease spread by air and one spread by a vector. [2 marks]

• Cue. Air: measles. Vector: malaria (by mosquitoes).

Q2. Suggest two ways the spread of a disease passed by direct contact could be reduced. [2 marks]

• Cue. Good hygiene (hand washing) and isolating infected individuals.