Immunoepidemiology can be defined as role of immunity in determining the occurance of diseases and epidemiological patterns of disease, in specified populations. In simple words, what immune parameters impact who gets sick and why.
In situations when immune system underperforms and does not clear aberrant cells, cancer can develop. More than 200 different cancers have been identified so far. What is known is that certain lifestyle choices increase risk of cancer, for example, smoking, excessive exposure to sun, obesity, but also some links of psychological disturbances have been made with cancer outcomes.
Immune system like any other system may improperly function due to inborn or acquired errors. For example, it can become inappropriately activated in allergies or in autoimmunity, or it can underperform due to immunodeficiencies.