Scientists discovered that the strain of Ebola responsible for the late 2013 outbreak in West Africa, the deadliest in the virus' history, may have mutated in the epidemic's early stages, allowing it to better target human hosts, Science Magazine reported Thursday.

The virus killed over 11,000 people and infected over 28,000 people mainly in Liberia, Sierra Leone and Guinea throughout the event, while previous outbreaks had never reached more than 600 people. Two scientific teams have now released papers containing evidence that the virus underwent a major mutation about three months into the epidemic that made it more suitable for human hosts as opposed to its traditional carrier, a species of fruit bat.

“The virus has never had this many human-to-human transmissions before, and there are a lot of mutations happening,” Pardis Sabeti, associate professor at Harvard University and co-author of one of the papers, told Scientific Magazine.

While analyzing 1,489 genomes taken from Ebola patients in West Africa in order to map out the virus' evolution and development, Sabeti's team, led by University of Massachusetts's Dr. Jeremy Luban, made an important discovery in a sample from March 31, 2014. The mutation, known as GPA82V, was the result of an amino acid change in the virus' surface protein cells, the area that allows it to infect hosts. The single alteration meant that the virus was now better adapted for primates such as humans. The new strain was also more fatal.

A second team reached the same conclusion completely independent from Sabeti and her team. University of Nottingham professor Jonathan Ball of the U.K. and Pasteur Institute researcher Etienne Simon-Loriere of France led their own investigation and found that the mutated virus was more likely to spread to humans.

Scientists believe that the West African Ebola virus epidemic began in late 2013 when an infected fruit bat bit a young boy in Guinea. Scientists believe that poverty, overcrowding and lack of access to sufficient medical care also contributed to the scale of the outbreak.

The crisis sparked fears worldwide of the virus spreading. Nations such as the Italy, the U.K. and the U.S. confirmed at least one infected patient within their respective countries. The virus, which causes severe bleeding, vomiting and high fever among other symptoms, has been officially declared "no longer a public health emergency" by the World Health Organization as of March 2016.