There are many causes of illness. One could get hit by a car, and the mechanical forces of the impact damage soft tissues and fracture bones. An analogous concept could be established for infectious diseases: a pathogenic microorganism “hits” the body and causes infection and inflammation along with fever, pain, and reduced physical fitness. Akin to the mechanistic forces that break a bone, the cause of infection is a virus or bacterium, and medical practice therefore tries to diagnose the causative pathogen in order to prescribe the correct therapy, be it an antiviral or an antibiotic. Moreover, in analogy to physical forces that cause injury, it is the number of microorganisms that have entered the body, which determines whether an infection will cause disease.
Different ways to become ill
But if we look at infection patterns in whole populations, it becomes obvious that not everybody who is infected by a specific “infective dose” of a particular pathogen becomes ill and shows clinical symptoms. In some cases, people appear immune even against high infective doses, whereas in other circumstances, even non‐pathogenic microorganisms can cause severe infections, which often affect leukemia patients or patients suffering from immunodeficiency. This highlights another crucial factor that determines whether we become ill or not: the immune system.
In contrast to an accident, infection depends on more than just physical forces. The immune system, involving numerous specialized cells, receptors, cytokines, antibodies, and so on, adds a layer of complexity that makes it more difficult to establish causality. Its efficiency in clearing the body of pathogens not only depends on its internal state—for instance the presence of memory B cells to produce specific antibodies—but also on external factors, such as the state of nutrition, climate, or stress levels. The relation between exposure to microorganism and infection is therefore not linear but complex or …
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