Susceptible hosts and risk factors

After an infectious agent gets inside the body it has to multiply in order to cause the disease. In some hosts, infection leads to the disease developing, but in others it does not. Individuals who are likely to develop a communicable disease after exposure to the infectious agents are called susceptible hosts. Different individuals are not equally susceptible to infection, for a variety of reasons.

Factors that increase the susceptibility of a host to the development of a communicable disease are called risk factors. Some risk factors arise from outside the individual – for example, poor personal hygiene, or poor control of reservoirs of infection in the environment. Factors such as these increase the exposure of susceptible hosts to infectious agents, which makes the disease more likely to develop.

Additionally, some people in a community are more likely to develop the disease than others, even though they all have the same exposure to infectious agents. This is due to a low level of immunity within the more susceptible individuals. Immunity refers to the resistance of an individual to communicable diseases, because their white blood cells and antibodies (defensive proteins) are able to fight the infectious agents successfully. Low levels of immunity could be due to:

• diseases like HIV/AIDS which suppress immunity
• poorly developed or immature immunity, as in very young children
• not being vaccinated
• poor nutritional status (e.g. malnourished children)
• pregnancy.

Finally, look back at Figure 1.2. We can now summarise the chain of transmission as follows:

• the infectious agent gets out of the reservoir through a route of exit
• it gets transmitted to a susceptible host by a direct or indirect mode of transmission and it gets into the susceptible host through a route of entry
• if it multiplies sufficiently in the susceptible host it will cause a communicable disease.