Steps in Replication
Modern Vaccine Strategies
Over the centuries, medical observers commented on a curious correlation: once people had had a particular infectious disease, they rarely got it again. In many cases, prior exposure to an infectious agent results in lifelong protection. As we now know, these are the hallmarks of the immune system-- inducibility, specificity, and memory. Obviously the best outcome wes exposure to a virus that caused only mild distress and still resulted in lifelong immunity.
Our immune system functions in two ways: it helps us recover from a virus infection, and then it protects us from additional occurences of that infection. Viruses that specialize in infecting vertebrate animals, like humans, with well-developed immune systems face the problem of having limited access to the host; they can infect each of us only once. Inevitably, becaus eof this limitation on reproductive capability, there is a selection for viruses that can get around this problem. Some destroy the immune system of the host(HIV); others have learned to hide in selected cells and recur periodically when the time is right( the herpesviruses); and some have developed mechanisms to change their coat proteins so that every exposure to the immune system is, in effect, a new event.(influenza A virus)
This constant evolutionary conflict between the host and its parasites, each seeking reprodutive advantage, ensures that new infectious agents will arise continuously. From a clinical perspective then, the goal is to find a way to expose individuals to a virus that doesn't cause disease and yet confers lifelong immunity. The first human disease for which this goal was achieved became, over three hundred years, an instance of the host's triumph.