Interactions

Diseases do not occur in isolation

A person may have more than one disease or health condition at the same time. These conditions can interact: one condition changes the risk, development or severity of another. The link may involve the immune system, cell damage, nutrition or the relationship between physical and mental health.

An interaction is more than two diseases simply occurring together. There must be a connection. For example, if one disease weakens a defence system, this can make a second disease easier to develop or harder to overcome.

It is useful to distinguish the broad disease types involved. Communicable diseases result from infection by pathogens and can spread between organisms. Non-communicable diseases are not transmitted between organisms and may be associated with genetic, environmental and lifestyle factors. Interactions can occur within either group or between the two groups.

A weakened immune system can allow other infections

The immune system normally helps to destroy pathogens. If it is defective or weakened, the body is less able to control infections, so the person becomes more likely to develop other communicable diseases.

HIV provides an important example. HIV infects and eventually reduces the number of particular lymphocytes involved in immune responses. As immune function declines, pathogens that would normally be controlled can cause serious opportunistic infections, such as pneumonia. The causal chain is:

HIV infection → fewer functioning immune cells → weaker immune response → greater risk of other infections.

Immune function may also be impaired by a person’s genetic make-up or by poor nutrition. This shows how an inherited factor or a dietary problem can alter susceptibility to communicable disease.

Some infections can contribute to cancer

Viruses reproduce inside host cells. Some viral infections can trigger changes in infected cells that contribute to the development of cancer, a non-communicable disease. For example, certain types of human papillomavirus, or HPV, can increase the risk of cervical cancer, while some hepatitis viruses can increase the risk of liver cancer.

This does not mean that every infected person develops cancer. Infection is a risk factor: other biological and environmental factors also affect whether cancer develops.

Immune responses can contribute to allergies

An immune response that begins during an infection can sometimes help trigger an excessive response to a normally harmless environmental substance. This allergy may then produce conditions such as skin rashes, hives or asthma.

Here, the pathogen does not directly cause the later allergic symptoms. Instead, the connection is through the behaviour of the immune system. This is another example of one health problem altering the likelihood of another.

Nutrition links several health problems

Malnutrition does not mean only starvation. It includes receiving too little food, too much food or the wrong balance of nutrients.

Too little energy or essential nutrients can cause deficiency diseases and weaken the immune system, increasing vulnerability to infection. At the other extreme, an unsuitable energy-rich diet can contribute to obesity, which is associated with a greater risk of cardiovascular disease, type 2 diabetes and some cancers. A dietary problem can therefore influence several conditions rather than producing one isolated effect.

Physical and mental health interact

Physical and mental health are closely connected. Severe or long-lasting physical illness can restrict everyday activity, cause pain and create uncertainty or isolation. It can therefore contribute to depression or other mental ill health. The mental health condition is a genuine part of the person’s overall health, not simply another name for the physical symptoms.

These examples show why health is better understood as a connected system. A pathogen, immune defect, nutritional problem or physical illness can begin a chain in which the risk or effect of another disease changes.

Describing an interaction clearly

A strong description identifies the starting condition, the change it causes and the resulting effect on another condition. For example:

HIV damages part of the immune system, so the body is less able to destroy pathogens; this increases the risk of opportunistic infections.

Words such as can, may and increases the risk are important. Disease interactions often alter probability rather than guaranteeing a particular outcome.

Core idea

Diseases and health conditions can interact: one condition may change the risk, development or severity of another. Do not treat diseases as if they always occur independently.

Essential interaction chains

• HIV infection → loss of important lymphocytes → weakened immune response → greater risk of opportunistic infections.
• Certain viral infections → changes in host cells → increased risk of particular cancers, such as HPV and cervical cancer.
• Infection-related immune response → excessive response to harmless substances → allergy, rash, hives or asthma.
• Poor nutrition → weaker immune system → greater susceptibility to infection.
• Unsuitable diet → obesity → increased risk of cardiovascular disease, type 2 diabetes and some cancers.
• Severe physical illness → effects on daily life and well-being → possible depression or other mental ill health.

Key distinction

Two conditions occurring together shows an association. To describe an interaction, state the biological or health-related link between them. Most interactions increase risk; they do not make the second disease inevitable.