AQA GCSE Biology: Communicable Disease

Communicable diseases are caused by pathogens that can spread from one organism to another. This guide explains what pathogens are, how bacteria and viruses actually make us ill, the ways diseases spread, and how that spread can be reduced. Understanding how pathogens spread is the key to understanding how to stop them.

Pathogens

Pathogens are microorganisms that cause infectious disease. They come in four types: bacteria, viruses, protists and fungi, and they can infect both plants and animals.

A disease is communicable if the pathogen can be passed from one organism to another. That is the whole difference between these diseases and the non-communicable ones in the health and non-communicable disease guide. Nobody catches coronary heart disease from a friend, but measles spreads across a classroom in a fortnight.

Being infected is not the same as feeling ill. The symptoms come from the damage the pathogen does, which is why the two groups below make a person ill in different ways.

How bacteria and viruses make us ill

Both reproduce rapidly inside the body, but the damage is done differently, and a question asking for the difference wants the mechanism rather than the name.

• Bacteria reproduce rapidly and release toxins, which are poisons. The toxins are what damage tissues, so a bacterial illness can be felt all over the body rather than only where the bacteria are.
• Viruses cannot reproduce on their own at all. A virus enters a cell and uses that cell to make copies of itself, until so many have built up that the cell bursts. The cell damage is the illness.

That difference matters beyond this topic: antibiotics kill bacteria but do nothing at all to a virus, because a virus is sheltering inside the body's own cells. The medicines and drug development guide takes that further.

How diseases spread

Pathogens can be passed on in several ways, and AQA expects the route to be named.

• Direct contact, for example touching contaminated surfaces or skin. Plants pass pathogens on by touching each other in the same way.
• Water, for example by drinking, cooking with or bathing in contaminated water. One infected supply can reach a whole population at once.
• Air, for example inhaling the droplets from a sneeze or a cough. Airborne spread is the hardest kind to stop, because breathing is not optional.

Some pathogens travel inside a vector, an organism that carries a pathogen from one host to another without catching the disease itself. The mosquito that carries the malarial protist and the aphid that carries a plant virus are both vectors.

Reducing the spread of disease

Every measure below works by closing one of those routes, and that is the link an answer needs to make.

• Hygiene, such as regular hand washing and disinfecting surfaces, closes direct contact. Treating a water supply does the same job for water.
• Isolating infected people keeps them away from everyone else, which closes both direct contact and air.
• Destroying vectors, for example using insecticides or draining the standing water mosquitoes breed in, removes the carrier rather than the pathogen.
• Vaccination closes no route at all. It removes the people a pathogen could infect, so an outbreak runs out of hosts, which is the subject of the vaccination guide.

If a question describes a situation and asks how to reduce the spread, name the route the pathogen is using and then give the measure that closes it. A measure on its own rarely earns both marks.

How this fits together

Knowing how pathogens spread sets up the specific diseases you need to know, caused by viruses, bacteria, fungi and protists, which are covered in the diseases caused by pathogens guide.