How one disease leads to another

1. Diseases do not act in isolation, because having one disease often makes a second disease more likely to develop or more severe when it does.
2. When a person has two or more diseases at the same time this is called comorbidity, and the combination is usually harder to treat than either disease alone.
3. There are four routes by which one disease opens the door to another.
   1. Damage to the immune system leaves the body unable to destroy pathogens it would normally deal with easily.
   2. A virus infecting host cells can disturb how those cells divide and so trigger a cancer.
   3. An immune response to a pathogen can leave the body reacting to harmless substances afterwards, producing an allergy.
   4. Severe physical illness can lead to mental illness, because pain, disability and uncertainty all take a toll.

HIV and tuberculosis

1. HIV, the human immunodeficiency virus, infects a group of white blood cells called helper T lymphocytes, also known as CD4 cells, which coordinate the body's response to infection.
2. The virus reproduces inside these cells and destroys them, so the number of CD4 cells in the blood falls steadily over several years while the number of virus particles rises.
3. Soon after infection the person has a short flu-like illness, after which they may feel well for years even though the virus is still reproducing.
4. Once the CD4 count drops far enough the immune system can no longer control pathogens, and the person is said to have AIDS, acquired immune deficiency syndrome.
5. Tuberculosis is a communicable disease of the lungs caused by the bacterium Mycobacterium tuberculosis, which spreads in droplets breathed out by an infected person.
6. In a healthy person white blood cells surround the bacteria and seal them inside small lumps of tissue in the lung, so the infection stays dormant and causes no symptoms.
7. In a person with HIV there are too few working lymphocytes to hold the bacteria in place, so the dormant infection breaks out into active tuberculosis with a persistent cough, weight loss and fever.
   1. This is why tuberculosis is the infection that most often kills people living with untreated HIV, even though HIV itself destroys no lung tissue.
8. Antiretroviral therapy slows the reproduction of HIV, so the CD4 count stays high, the immune system keeps working and the risk of tuberculosis falls sharply.

HPV and cervical cancer

1. Human papillomavirus, usually shortened to HPV, is passed on by skin to skin contact during sexual activity and infects the cells lining the cervix.
2. Most HPV infections are cleared by the immune system within about two years and cause no lasting harm.
3. A few strains of HPV insert their genetic material into the DNA of the cervix cells and make proteins that switch off the controls on cell division.
4. The infected cells then divide uncontrollably, forming a mass of abnormal cells that can develop over many years into cervical cancer.
5. The cancer itself is a non-communicable disease, because the tumour cannot pass to another person, yet the virus that started it is communicable.
6. Cervical screening finds abnormal cells before they become cancerous, and vaccination against HPV before a person becomes sexually active stops the infection that starts the whole sequence.

Other interactions between diseases

1. The immune response raised against a pathogen can leave the body over-reacting to harmless substances afterwards, which is why some children develop asthma or skin rashes following a viral infection of the airways.
2. Physical damage to a barrier lets pathogens reach tissues that are normally sealed off, so a deep wound or a burn is frequently followed by a bacterial infection of the skin.
3. A non-communicable disease can also make a communicable one more likely, because poorly controlled diabetes reduces blood flow to the feet and slows healing, so small wounds there become infected.
4. Long or severe physical illness raises the risk of depression and other mental illness, because of continuing pain, loss of independence and the strain of an uncertain future.
5. For all these reasons a doctor treats the whole patient rather than one illness in isolation, and controlling the first disease is often the most effective way of preventing the second.