Mr. Gaines has dentures, but he says that they give him pain so he often does not wear them while eating. It takes him a long time to finish his meals, and he has to concentrate so hard on chewing his food that he does not seem interested in conversing with anyone around him.

Which of the following best reflects what we know about how the flu virus moves between species? a) the flu virus in a pig is mutated and replicated in alternate arrangements so that humans who eat the pig products can be infected. b) a flu virus from a human epidemic or pandemic infects birds; the birds replicate the virus differently and then pass it back to humans. c) an influenza virus gains new sequences of dna from another virus, such as a herpesvirus; this enables it to be transmitted to a human host. d) an animal such as a pig is infected with more than one virus, genetic recombination occurs, the new virus mutates, the virus is passed to a new species such as a bird, and the virus mutates again and can now be transmitted to humans.