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The World Heath Organisation (WHO) is responsible for advising on ways to improve heath around the world. They define health as a state of complete physical, mental and social well-being. This means that being free from disease alone is not necessarily enough to be in good health.

Different types of diseases

Diseases can be grouped into two types:

• communicable, which can be transferred from one person to another, or from one organism to another, eg in humans, these include measles, food poisoning and malaria.
• non-communicable, which are not transferred between people or other organisms

Interactions between different types of diseases

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 infections, known as HIV, which leads to AIDS, affect the immune system and lead to an increase in susceptibility to 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.