Defects in the immune system mean that an individual is more likely to suffer from infectious diseases.
For instance, human immunodeficiency virus (HIV) infections, which lead to AIDS, affect the immune system and lead to an increase in infectious diseases.
Viruses living in cells can be the trigger for cancers. For instance, the majority of cases of cancer of the cervix are linked with a virus present in the female reproductive system.
The reaction of the immune system to pathogens and other foreign bodies can trigger allergic reactions that lead to skin rashes and asthma. For instance, severe respiratory infections in babies can lead to asthma in later childhood. Severe physical ill health can lead to depression and other types of mental illness.