Different types of disease may interact. This can mean that the presence of one disease can lead to a higher chance of developing another disease.
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 risk of developing other 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.
Anything that increases a chance of developing a disease is called a risk factor. Risk factors can be caused by lifestyle factors or substances in a person's body or their environment. Some diseases are caused by an interaction between risk factors.
Some risk factors have been proven to cause a disease. We know that smoking causes lung cancer. Other risk factors are linked but not proven.
Some diseases, their effects and their risk factors are shown in the table below.
Type 2 diabetes
Blood sugar levels cannot be regulated properly
Scar tissue is formed in the liver which stops it removing toxins
Smoking causes lung cancer and also the underdevelopment of unborn babies