Disease and Infection

Microbes and Pathogens

Microbes (an organism too small to be seen by the naked eye) can enter the body through the mouth, nose and cuts or bites in the skin

Microbes (bacteria and viruses) can cause diseases. There are billions of microorganisms inside us and on our skin. Over 95% of those inside us right now are helpful - or at least harmless. The rest are not nice to have around - they may multiply rapidly inside our bodies and cause infectious illness; these nasty ones are called pathogens.

Pathogens cause us problems by either:

  • releasing toxins (poisons), or
  • invading and damaging our cells (if they are viruses)

Examples of diseases pathogens cause are

Pathogen Disease
Bacteria Cholera
Viruses Influenza (flu)
Fungi Athlete's Foot
Protozoa Malaria

To a microbe, a human being is a bit like a luxury hotel on legs, so it tries to get inside our bodies. The body has two ways of defending itself.

  • The first line of defence is to keep the pathogen out. The body is like a fortress: the outer defences include our thick skin, sticky mucus and cilia in the respiratory system, enzymes in tears - and acid in the stomach which kills most pathogens that enter our stomachs via our food.
  • The second line of defence kicks in if a pathogen does manage to get through via the normal openings or via a cut or graze. What happens is that our white blood cells attack the microbes in various ways - and usually manage to destroy them e.g. providing a scab for protection

Two ways in which doctors help the body's fight against infection are:

  • Vaccination, in which pathogens are introduced into the body in a weakened form. This process causes the body to produce enough white blood cells to protect itself against the pathogens, while not getting diseased.
  • Antibiotics, which are effective against bacteria but viruses. Some strains of bacteria are resistant to antibiotics.



Some diseases are not caused by pathogens and so are not infectious. For example:

  • scurvy is caused by vitamin C deficiency
  • anaemia is caused by iron deficiency
  • diabetes and cancer are disorders of the body.

Some disorders are inherited, such as red-green colour vision deficiency.


A cancer happens when cells begin to divide out of control. They form tumours that can sometimes be felt as an unusual lump in the body. Diet and lifestyle can increase the risk of developing certain cancers. For example:

  • smoking increases the risk of lung cancer
  • using sunscreen reduces the risk of skin cancer
  • eating more fruit and vegetables reduces the risk of bowel cancer.

Tumours can be benign or malignant,

  • benign tumours grow slowly and are usually harmless – warts are benign tumours
  • malignant tumours often grow more quickly and may spread throughout the body.


Parasites are organisms that live on or in a host organism. The parasite benefits from this arrangement, but the host suffers as a result. For example, tapeworms are parasites. They live inside another animal, attaching themselves to the host’s gut and absorbing its food. The host loses nutrition, and may develop weight loss, diarrhoea and vomiting. Parasites do not usually kill the host, as this would cut off their food supply.


Malaria is a disease caused by a protozoan, a type of single-celled organism. The malaria parasite is spread from person to person by mosquitoes. These insects feed on blood and the malaria parasite is passed on when the mosquito takes a meal. Organisms that spread disease, rather than causing it themselves, are called vectors. The mosquito is the vector for malaria.

It is important to understand the life cycle of a pathogen and the way in which vectors spread the pathogen. This knowledge gives scientists ideas about how the spread of a disease might be controlled. In principal, if the life cycle of a pathogen can be broken, eventually all the individuals of that pathogen will die out, leaving a disease-free population.

For example, the spread of malaria can be controlled by avoiding contact with the vector. One way to do this is to avoid being bitten by mosquitoes, for example using mosquito netting at windows, doors and around beds. The mosquitoes may be killed using insecticides. The parasite itself can be killed by giving infected people drugs such as Lariam.


sign up to revision world banner