The size and range of populations of ticks carrying different diseases can expand like volcanic eruptions. They seem to be impossible to predict. There is a lot more that we don’t know than we do. The tick-borne Powassan virus, for example, discovered in 1958, still affects people in tiny numbers. The bad news is that, like virtually all other tick-borne diseases, it may well have the ability to flare up like Kilauea. And as the CDC notes, Powassan can infect the central nervous system and cause meningitis and encephalitis, which can be fatal, and it can lead to long-term neurologic problems.