H7N9 Influenza seems to have settled somewhat for the time being…..(famous last words)
Novel CoronaVirus (NCoV) infection is continuing to cause problems with recent evidence of human to human spread. Click here for an update. See below for a bit about how coronavirus replicates.
These emerging infections made me ask myself a couple of questions. Why are they usually viruses and why do they sometimes just go away as quickly as they came…?
Almost all emerging viruses come from an animal (zoonotic) reservoir (for H7N9 this is birds and for NCoV probably bats). Mutations (NCoV) or genetic reassortments (H7N9) will occasionally produce a virus that is capable of jumping the species barrier from animal to human. The new “human” virus has no natural immunity so is often very pathogenic (H7N9 and NCoV), but is not used to jumping from human to human so transmissability is often low (H7N9 and NCoV), particularly in the early stages.
This is not always the case however. In contrast the H1N1 2009 pandemic managed to produce a virus that was of low pathogenicity but of high transmissability (the random odds of genetic reassortment…).
When the virus first starts replicating in humans it then undergoes a period of adaptation to it’s new host, with further selection of mutant strains etc. This is a very difficult period to predict exactly how the virus is going to change and how its behaviour will be affected. Both H7N9 and NCoV are probably still in this phase so it is a bit of a guessing game as to the eventual role of these viruses in the human host (hence the need for very close surveillance). In some cases the virus simply decides it doesn’t much like humans as a host and disappears.
So why is it viruses and not usually bacteria that cause emergent diseases?
I don’t know if I really know the answer to this one, but I suspect the solution lies in the relative complexity of bacteria compared to viruses. Because viruses are genetically more simple micro-organisms a single mutation or genetic reassortment is more likely to have a more dramatic effect on behaviour with the potential of creating a change that is able to facilitate that species jump. On an evolutionary timescale, bacteria are probably a bit lazier, finding a comfortable niche within a certain species and reluctant to go anywhere on the basis of spontaneous mutations alone.
It’s a complex area, and one I would not claim to have any degree of expertise in. However I do think that it is important for us as microbiologists to have at least a basic understanding of the different factors at play. It also makes following the progress of NCoV and H7N9 just that little bit more interesting…