Communicable (infectious) diseases: Students should be able to explain how the spread of diseases can be reduced or prevented.

The idea

Spread of disease depends on a pathogen leaving one host, travelling, and entering a new host. Breaking any link reduces transmission. Direct contact is limited by hand-washing, covering cuts, using condoms and isolating infected people. Water-borne spread is reduced by clean water supplies, sewage treatment and chlorination. Airborne spread is reduced by ventilation, covering the mouth when coughing and using masks. Food-borne spread is reduced by refrigeration, cooking thoroughly and hygiene. Vectors such as mosquitoes are controlled by nets, insecticides and draining standing water. Vaccination makes hosts resistant, so the pathogen cannot multiply and pass on. In an exam, name the transmission route, then match a control method to that route and explain why it works.

What earns the marks

• Identify the transmission route first: direct contact, water, air, food or a vector, because the control method must interrupt that specific route.
• Hand-washing, disinfecting surfaces and covering cuts reduce direct contact and fomite transmission by removing or containing pathogens.
• Clean water, sewage treatment and chlorination reduce water-borne diseases such as cholera by killing or removing pathogens before ingestion.
• Ventilation, covering the mouth when coughing and masks reduce airborne droplet transmission by diluting or trapping droplets.
• Refrigeration, thorough cooking and hygienic food handling reduce food-borne illness by slowing or killing pathogen growth.
• Vaccination and isolating infected individuals reduce spread by making hosts resistant or preventing pathogen release to new hosts.
• Vector control, such as mosquito nets, insecticides and draining standing water, reduces diseases spread by insects.

Common mistakes and how to put them right

• Saying 'kill germs' without naming the route or method.

  Put it right: State the route and the specific control, for example chlorination kills water-borne pathogens.

• Confusing a treatment with prevention.

  Put it right: Antibiotics treat bacterial infection but do not prevent spread, whereas vaccination prevents infection.

• Assuming all diseases spread the same way.

  Put it right: Match each control to its route, such as masks for airborne spread and clean water for water-borne spread.

Exam tips

• Use the chain-of-infection idea: pathogen leaves host, travels, enters new host; then explain which link your method breaks.
• Give a named disease and its route where possible, for example cholera via water or influenza via air, to make the explanation concrete.
• For 'explain' questions, write a because clause linking the method to reduced transmission, not just a list of actions.