This would fit in with the month of birth being significant, if certain viruses are more prevalent at certain times of year.
When I worked in psychology there was a theory around about schizophrenia, some research again showing it may have a viral link, but the child had to have pre-existing genetic tendencies to be affected by the virus. Again there were certain months which had high levels of sufferers with their birthdays. This research was from a long time back, and not being in the field any more don't know whether it was upheld, and too lazy to look!
Also a colleague who had ms came from a locality where about a third of the children in her class at school went on to develop the disease as adults. The prevalence in her part of the country was also extremely high as a percentile.
Maybe there is something in the idea of susceptibility triggered by a viral event? When I stop being depressed I still have my access to med journals online from my old job, and will get it renewed again in a few weeks, so remind me and I'll look into it. But Quix is way ahead of me on this one I'm sure.
Interesting. Yes, I think I have some data on retroviruses. I'll look.