Generally, immune systems mount responses only against pathogens that have already infected the bodies they are protecting. Science, though, can shorten the path to immunity by vaccination. This involves presenting the immune system with harmless or lookalike versions of dangerous pathogens so that it may create antibodies and killer cells hostile to the real thing in advance of any actual infection, thereby reducing its danger. Like immune responses themselves, however, vaccination generally has to wait for the appearance of the pathogen in question before it can do its stuff. Do you think that Predicting viral evolution may let vaccines be prepared in advance?