Traces of Yersinia pestis, the plague-causing bacterium, have been recently found in the teeth of skulls from the Bronze Age.
Plague is being spread by fleas through rats and other animals, and immediate treatment with antibiotics is needed to contain it.
By acquiring a single gene, the bacteria could also pneumonic and bubonic plagues.
One pet dog began to transmit plague to four persons in Colorado, and died while the four humans got cured.
Harsh winters the country experienced did not kill ticks as snow insulated larva and helped them survive.
A new study has arrived at conclusions based on analysis of climate preceding the outbreaks.
