There are myriad ways that infectious agents — viruses, bacteria, parasites, fungi and prions — can do long-term damage to the host, directly or indirectly.
For instance, damage from the initial infection may cause a cascade of host responses that result in pathology even after the acute infection has resolved.
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Alternatively, microbes can infect us and then linger in our bodies for months or years as either a persistent active infection or a persistent nonreplicating (latent) infection.
These infections can cause long-term damage via several different means, including inducing inflammation that leads to tissue destruction, or reactivation of a latent pathogen that begins reproducing again in times of stress.
Currently there are numerous competing theories for how the SARS-CoV-2 virus may cause long-term symptoms.
PASC [or long COVID] may be a hit-and-run infection, potentially triggering inflammation that could do damage, or it could cause direct damage to tissues due to replication of the virus in targeted organs.
It’s possible the virus may linger in reservoirs within the body, but there has been no prior documentation of persistent infection of the host by human coronaviruses, so this remains unproven.
