Disease Interactions

How one disease can influence another

Diseases do not always act independently. One disease can change conditions inside the body so that another becomes more likely to develop or more severe. For example, damage to the body's defences can allow another infection to become active. An infection can also disturb the controls on cell division and lead to cancer.

These are different kinds of interaction. HIV and tuberculosis show how one communicable disease can increase the risk from another. HPV and cervical cancer show how a communicable infection can lead to a non-communicable disease.

HIV weakens the defence against tuberculosis

The immune system protects the body against pathogens — disease-causing microorganisms. White blood cells are important parts of this defence. Some, called lymphocytes, help coordinate the response to infections.

HIV, the human immunodeficiency virus, infects and damages particular lymphocytes. Without treatment, this damage can build up over time, leaving the immune system less able to control other pathogens. Severe damage can lead to AIDS, acquired immune deficiency syndrome, when the person becomes vulnerable to serious infections.

Tuberculosis (TB) is caused by a bacterium, not by HIV. It usually affects the lungs. A person may carry TB bacteria without being ill because their immune system keeps the bacteria under control. This is a dormant, or latent, infection.

If HIV weakens that person's immune system, it may no longer keep the TB bacteria under control. The bacteria can multiply and cause active tuberculosis, with symptoms such as a persistent cough, fever and weight loss. HIV therefore increases the risk that a TB infection will develop into active disease, and TB can become a serious threat to someone with weakened immune defences.

The link is not that HIV creates TB bacteria. The person must also be infected with those bacteria. Instead, HIV changes the body's ability to resist them. Treatment that slows HIV reproduction helps preserve immune function and reduces the risk of active TB.

HPV can lead to cervical cancer

HPV, human papillomavirus, is a group of viruses passed on through intimate skin-to-skin contact, including during sexual activity. Some types can infect cells in the cervix, the lower part of the uterus that opens into the vagina.

Most HPV infections are controlled or cleared by the immune system and do not cause cancer. However, a persistent infection with a high-risk type of HPV can interfere with the normal controls on cell division in cervical cells.

Abnormal cells may then develop and, over many years, can become cancerous. Cancer involves uncontrolled cell division. The important sequence is therefore: persistent high-risk HPV infection → abnormal changes in cervical cells → possible cervical cancer.

HPV infection is communicable, but cervical cancer is non-communicable. The virus can pass between people; the cancer itself cannot. An infectious cause does not make the resulting cancer something another person can catch.

Comparing the two interactions

In the HIV–TB interaction, the first infection weakens the body's defence against a second pathogen. In the HPV–cancer interaction, a virus changes how the body's own cells behave.

These links also explain why preventing or controlling one disease can reduce another. HPV vaccination helps prevent infection with targeted HPV types and so reduces the risk of cervical cancer. Cervical screening can identify high-risk HPV or abnormal cell changes, allowing follow-up and treatment of precancerous changes before cancer develops.