COVID-19 can sometimes overwhelm young and old immune systems and in some cases can result in ‘Severe Acute Respiratory Syndrome’ pneumonia and death (CDC (2020). The risk is greater for older people who may simultaneously be dealing with cancers, heart disease, diabetes, emphysema or other health issues. As we age the immune system deteriorates (immunosenescence) which reduces the response of the adaptive immune system that needs to respond to the virus infection (Aw, Silva & Palmer, 2007; Osttan, Monti, Gueresi, et al., 2016).
Severity of disease may depend upon initial dose of the virus
In a brilliant article, How does the coronavirus behave inside a patient? We’ve counted the viral spread across peoples; now we need to count it within people, assistant professor of medicine at Columbia University and cancer physician Siddhartha Mukherjee points out that severity of the disease may be related to the initial dose of the virus. Namely, if you receive a very small dose (not too many virus particles), they will infect you; however, the body can activate its immune response to cope with the infection. The low dose exposure act similar to vaccination. If on the other hand you are exposed to a very high dose then your body is overwhelmed with the infection and is unable to respond effectively. Think of a forest fire. A small fire can easily be suppressed since there is enough time to upgrade the fire-fighting resources; however, during a fire-storm with multiple fires occurring at the same time, the fire-fighting resources are overwhelmed and there is not enough time to recruit outside fire-fighting resources.