Flu vaccines need to be revised and changed every year because the flu virus constantly mutates. Flu viruses can change in two ways: drift or shift. Antigenic drift occurs when genes of the virus slowly change during replication until the immune system no longer recognizes it. On the other hand, antigenic shift occurs when a major change in the virus happens. This usually leads to a totally different kind of virus. This is what happened during the H1N1 scare in 2009.
DNA viruses like mumps are less prone to mutation due to the presence of DNA polymerase. RNA viruses are more prone to mutation because they undergo reverse transcription to simulate the role of DNA.
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Epidemic diseases in the New World started when Cortez went to conquer the Aztecs and the Inca and he ended up spreading disease killing thousands. They were spread by the rise of agriculture and through trade. The carriers of these diseases were bird, humans, and animals. Diseases included syphilis, tuberculosis, flu, and typhoid fever.
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