There are some nasty infections out there and one of them is the plaque. Nowadays very rare, the plaque killed off about half of 14th century Europe. Granted, in those days effective treatments for just about anything were far and few between. Still, it gives one pause as to why this infection appeared suddenly and was so deadly.
Full blown plaque is truly deadly. It can infect the lungs and from there spread almost anywhere in the body. Pneumonia, kidney failure, sepsis and in short total body shutdown can result. Even today deaths are caused by this disease.
Yersinia pestis, is the bad boy bacteria responsible for the plaque. A recent study cast light on the incredible potential these and other deadly bacteria have in terms of mutating from a organism causing minor problems to a deadly attacker.
Some Yersinia bacteria lack a gene which produces a protein named rather simply Pla. These bacteria only make your stomach upset and perhaps a little vomiting and diarrhea as well. If the gene that makes Pla is reinserted into the Yersinia bacteria, then all hell breaks loose. Even worse, if the gene evolves further then the bacteria can invade deep tissues as well, which results in bubonic plague.
Bottom line is that evolution is a powerful thing. Minor changes in bacteria can lead to major problems for humanity.