Cholera has been disappearing and reappearing for years. Now in it's seventh pandemic, scientists are studying the DNA of historical samples to reason with how the harmless version transformed into the deadly version. The bacterium, Vibio cholerae, leads to diarrhea and therefore dehydration and eventual death. The sickness spreads through contact with bodily fluids. The illness spreads so rapidly that it is often difficult to help other countries in time.
Evolutionary maps of the cholera sample's DNA over time were made just as any other evolutionary tree would be made. Seven stages of the disease were identified in which number seven came from one through six. Each of the six strains were categorized using their toxicity and the rate at which they spread. One of the most pivotal stages of the bacteriums evolution was the gain of the tcpA gene that codes for hairlike structures that allow the bacterium to cling inside the intestine. It was not until 1903/1908 that the cholera strain caused disease in humans. Once it can affect humans, a phage was able to infect the bacterium and pick up the cholera toxin gene causing diarrhea. The disease now how spread through water supply but it was still not considered a pandemic. The true cause of the pandemic is unknown and humans would have to be studied to see how fast a disease travels amoung others.
The cholera pandemic reminds me of the bacterium that become antibiotic resistant. Bacteria will "find a way", in a sense, to thrive. The antibiotics were killing off the bactera until some of the bacteria that had antibiotic resistance lived and those genes were passed on. Similarly, the more toxic genes of cholera have been passed on which has led to the pandemic.
Sources:
http://www.sciencemag.org/news/2016/11/how-today-s-cholera-pandemic-was-born
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