Infectious diseases are caused by microorganisms; other factors may determine an individual’s susceptibility to infection, but the microorganism is still the cause
Factors like nutrition, genetics and age can influence an individual’s susceptibility to a microorganism, such as a virus or a bacteria. These factors make individuals more or less likely to develop infections after they were exposed to a microorganism. That being said, even individuals who are young and healthy can develop infectious diseases because the cause of infectious diseases is still the microorganism.