What would you think if your 7th grade history teacher told you the Black Death was just a tummy bug? Well, he would be right. Nature Communications published a study on Tuesday that explains how the bacterium Yersinia pestis, which is responsible for the plague, was originally a stomach bacteria and it wouldn’t be deadly if it hadn’t obtained two significant genetic mutations.

As it turns out, there are three plagues caused by Y. pestis—pneumonic, bubonic and septicemic, The Washington Post reported. The pneumonic plague tends to be the most fatal of the three. Although it is rare, when a case occurs, it often goes undetected for a long period of time because symptoms are similar to the flu. The emergence of this pneumonic plague from a once strictly-tummy-bug is what got scientists so curious.

Microbiologists at Northwestern University Feinberg School of Medicine in Chicago tested strains of the bacteria in mice to determine what exactly occurred during this deadly evolution and what mutations made this possible.

Although the specific time is not yet known, at some point, Y. pestis was infected with a gene from other bacteria. The infection mutated the Y. pestis Pla gene to change how it was expressed. Upon original mutation, it created a strain responsible for the lung-infecting pneumonic plague. Later on, it suffered another mutation, which led to a newer bacterial strain, which is responsible for the bubonic plague.

As the Pla gene continued to develop with the mutation, it allowed the bacterium to infect the body at a faster and more efficient rate. The researchers found that this mutation happened early on in the bacterium’s evolution and that it is what enabled for the bacterium to infect the lungs. Based on research of the bacteria that lacked the Pla, without that particular gene, the bacterium would be able to invade the lungs and perhaps colonize, but not cause serious illness.

The quick evolution to a mutated strain and the efficiency of that strain led to the epidemics. Nobody had experienced the illness prior to the mutation because it was so quick to adapt. Ultimately, the efficiency of the bacterium to adapt to its own circumstances is what enabled the diseases to spread so vastly and suddenly.

Researchers said their data suggests how the small, sudden mutation allowed for rapidly evolving strains of diseases. This particular bacterium was very susceptible to mutation, but that is not necessarily true for all microbes. The microbiologists in this study don’t expect another mass-killing bacteria to develop anytime soon, but they do plan on doing more research to further understand what causes the mutations and deadly evolutions.

In other plague related coverage here at Immortal News, a group of scientists believes that the bubonic plague outbreak of the 1300s, the Black Death, may have been spread by gerbils, not rats.