Many years ago, I came across the novel notion that it was possible that humans were meant to harbor specific viruses, just like we house beneficial bacteria and helminths.
This week, I was really excited to spot an article called “Viruses of the Human Body”[i] which expands upon this idea. It is a fantastic summary of where research on the virome currently stands.
Using a variety of DNA and RNA sequencing techniques, scientists are rapidly expanding the National Center for Biotechnology Information and the European Nucleotide Archive, public databases of genetic information. As more and more resident viruses are discovered, the question becomes: is this new virus a pathogen or not?
This is actually a pretty complex question because, as it turns out, most viruses are “neither consistently pathogenic nor always harmless, but rather can result in different outcomes depending on the health and immunological status of their hosts.”
That is, while some viruses we know are always harmful, some are harmless while we are healthy. They remain dormant in us for years, causing no issues until we become immune suppressed (from either physical or mental stresses). Perhaps the best known of these are the herpesviruses, which cause everything from cold sores (so called, as they are associated with the immune upheaval caused by the common cold) to shingles (often associated with severe mental or physical stress) to even forms of cancer.
Then there are those that are actually beneficial and, in fact, support normal development of the human immune system.
“Viral infections at a young age may help our immune system develop properly, providing protection against later infections and preventing immune overreactions that lead to allergies. Viral infections of the respiratory and gastrointestinal tracts of healthy infants are now known to be common and often asymptomatic, likely thanks to protection by maternal antibodies delivered across the placenta and via breast milk. Such attenuated infections might provide a form of natural vaccination against later infections with related, more-pathogenic viruses. Just as the proper development of the human gut and immune system in infants is dependent on the presence of a bacterial gut microbiome, a recent study found that early enteric viral infection could have a similar beneficial effect in mice.”
Believe it or not, there are also viruses that confer protection against other pathogenic viruses. A virus in the same family as hepatitis C (and Zika and dengue) mitigates the effects of the HIV virus. Those infected with HIV and pegigvirus C live longer than those without the latter.
Perhaps most amazing of all, the human genome itself is about 8% retroviral DNA sequences that have inserted themselves and now “serve essential functions for their host’s survival and development.” Who knew that we are literally 8% viral?!
We know that our resident bacterial microbiome and our macrobiome interact in a variety of ways that benefit we hosts. I really do wonder how the virome affects both. I am quite sure we’ll be reading more about this research in the coming years.