The (+) ssRNA genome of poliovirus, a single long RNA molecule, is unable to recombine with (+) ssRNA genomes from other related picornaviruses by the process of crossing over. Crossing over requires a set of highly specialized enzymes that are not available to the viruses. This means that there are only a very small number of closely related poliovirus strains and immunity against one of these strains confers immunity against the others. Influenza virus presents a dramatically contrasting example. The influenza virus genome is composed of different RNAs, each carrying different genes
When two different types of flu virus simultaneously infect a single host cell the two viral genomes may recombine by simply exchanging RNAs. A progeny virion released from this cell can have RNAs from each different infecting virus and thus a new unique strain has been created. This has devastating consequences on human health
The natural reservoir for many flu viruses is the shore bird population around the world, which includes migratory ducks and geese. At least a dozen distinct strains of flu virus live continually in these birds. In rural China and other parts of the world, ducks and pigs are kept in close proximity with one another and with humans. The ducks become infected with a flu virus originating in the wild shore birds. They can then pass the virus to a pig, which is also readily infected by human flu viruses. In the infected cells of the pig, the two viral genomes can mix and RNAs from both viruses can be packaged into one virion. In this way, a new recombinant flu virus, one quite different from pre existing strains may emerge and infect humans
The exposed humans would have not experienced this strain before and thus there is no immunity to it. The flu virus now spreads throughout the population creating a worldwide epidemic. These pandemics create serious respiratory infections but usually prove lethal to only a few mostly the weak and elderly
This is not always the case, however. The flu pandemic of 1918 killed 20 million people in Europe in a matter of months and perhaps 100 million throughout the world. Many of those affected were previously young and healthy. Why some strains of flu virus are benign while others are lethal is not understood though it clearly depends upon which viral RNAs are present in the recombinant flu virus. Modern antibiotics, which are highly effective against bacterial infections offer no protection against viral infections
When two different types of flu virus simultaneously infect a single host cell the two viral genomes may recombine by simply exchanging RNAs. A progeny virion released from this cell can have RNAs from each different infecting virus and thus a new unique strain has been created. This has devastating consequences on human health
The natural reservoir for many flu viruses is the shore bird population around the world, which includes migratory ducks and geese. At least a dozen distinct strains of flu virus live continually in these birds. In rural China and other parts of the world, ducks and pigs are kept in close proximity with one another and with humans. The ducks become infected with a flu virus originating in the wild shore birds. They can then pass the virus to a pig, which is also readily infected by human flu viruses. In the infected cells of the pig, the two viral genomes can mix and RNAs from both viruses can be packaged into one virion. In this way, a new recombinant flu virus, one quite different from pre existing strains may emerge and infect humans
The exposed humans would have not experienced this strain before and thus there is no immunity to it. The flu virus now spreads throughout the population creating a worldwide epidemic. These pandemics create serious respiratory infections but usually prove lethal to only a few mostly the weak and elderly
This is not always the case, however. The flu pandemic of 1918 killed 20 million people in Europe in a matter of months and perhaps 100 million throughout the world. Many of those affected were previously young and healthy. Why some strains of flu virus are benign while others are lethal is not understood though it clearly depends upon which viral RNAs are present in the recombinant flu virus. Modern antibiotics, which are highly effective against bacterial infections offer no protection against viral infections