Re: Are there any examples of viruses beneficial to their host?

Excellent question!

Whenever you hear someone say that, you can be pretty sure the answer is going to be a long-winded version of "I don't know the answer." ;)

So let me just start off that way. I don't know the answer. But I'm willing to give some reasonable guesses anyway, so you'll have more excellent questions in the future.

The first thing to consider, is if you want to find a virus that is beneificial to the individual, or beneficial to the host speicies as a whole, or a group of individuals in the host species. The book "Virus X" by Frank Ryan has an excellent theory about how viruses might be beneficial to a host speicies even if the virus harms each infected individual. It goes basically like this: The virus does little horm of no harm to individuals of the host species, but is beneficial to the species overall because it does greater harm to competing species. For example chimpanzees might carry an immunodeficiency virus that doesn't hurt chimps very much, but the virus can be transmitted to gorillas and other speicies that come in contact with the chimps (by fighting for example) and it does great harm to these other species. As far as I know, this type of theory has not yet been pfoven to be true, but it makes so much sense that there are likley to be many examples where it will be proven true in the future.

It is easy to say that cowpox virus is beneficial to humans because infection with cowpox virus provides cross-protection to smallpox virus. The first real vaccine was made by infecting humans with cowpox virus. But the cowpox does cause some pox, and it does not help humans that don't get exposed to smallpox in the future. And humans are not the real natural host of cowpox, so I'm not sure that this is the answer to your question.

A curious virus that I am aware of is the bacteriophage (phages are viruses that infect bacteria) that infects Corynebacteria diphtheriae. The phage does not harm the bacteria vey much, as far as anyone can tell, but it causes the bacteria to kill the bacterial host (humans and other animals). It is not clear how harming the host would help the bacteria or the phage that infects the bacteria. But it would be an interesting topic to explore, and it is related to your question.

It would not be surprising to find viruses that help their host, nor surprising that we haven't yet discovered them even if they do exist. Viruses are difficult to study, so we only spend time so far, studying the ones that cause great harm. The ones that hurt us are easier to detect (it is easier to see that someone is sick, than to see that someone is extra healthy) for one thing. Viruses like HTLV-I which harm only a small percentage of infected people, were only recently discovered.

Brian Foley, PhD