This is a question without a good answer. The obvious response is that more research is needed. That said, I'm curious as to what people's opinions are based on their own experience and their understanding of the research. Some things to consider: Whether the triggering insult remains as an ongoing factor or if leaves a common dysfunctional aftermath If an initial infection or trauma causes lasting damage in its acute stage (e.g. encephalopathy, myocarditis) A given pathogen's tropism for a type of tissue or organ system (e.g. enterovirus vs herpes family) Treatability of a given infection with drugs (e.g. antibiotics vs antivirals) Ability of a pathogen or triggering insult to cause autoimmunity / immune dysfunction (through either molecular mimicry e.g. a pathogen is misidentified as a host protein, or chronic immune evasion / activation) Ability of a pathogen to evade the immune system (e.g. by forming intracellular non-cytolytic infections, hiding in biofilms or "immortal" tissues, using cytokines like IL-10 to evade detection) If symptom / severity / duration variability can be explained by other factors such as host constitution / genetics, chance, sex, age, and/or ethnicity.