I think it would be over-exaggeration to think that there are millions of viruses ready to jump on us and bring us back to the 14th century. That would be looking over a ledge that isn't there.
The launch of phase 1 Ebola vaccine studies is a first step in developing a vaccine that could be licensed and used in the field to protect not only the front line health care workers but also those living in areas where Ebola virus exists.
The nature of a protective immune response to HIV is still unclear. Because in a very, very unique manner, unlike virtually any other microbe with which we're familiar, the HIV virus has evolved in a way that the immune system finds it very difficult, if not impossible, to deal with the virus.
When you put someone on therapy, you lower the level of virus such that it makes it very difficult for them to infect others.