The risk of a severe illness increases steadily with age, in particular from 50 to 60 years. Older people can become more seriously ill upon infection due to weak immune system function. Since non-specific symptoms of the disease, such as fever, are, in fact, the immune system's response to the infection, they may be weaker or fail to work in older age, which means that the patient usually goes to the doctor in later stages.
Various underlying diseases such as cardiovascular diseases, diabetes, diseases of the respiratory system, liver and kidney, and cancer appear to increase the risk of developing a severe illness regardless of age.
In older people with pre-existing underlying diseases, the risk of developing a severe illness is higher compared to those in which only one factor (age or underlying condition) is present; if there are several underlying illnesses, the risk is likely to be higher than the case suffering only one underlying disease.
There is a higher risk for patients with suppressed immune systems (e.g. due to a disease that is associated with an immune deficiency or taking medications that suppress the immune system, such as cortisone). It is not yet sufficiently known which combination of risk factors with other (life) circumstances represent a particularly high risk for a severe course of the disease in COVID-19.
According to current knowledge, pregnant women do not seem to face an increased risk compared to other women. Severe case have been rarely seen in children so far.