As ticks expand their habitat due to global warming, the number of people affected by Lyme disease has increased exponentially since the 1990s. For instance, from 1997-01, only 330 Lyme disease cases were reported, while 3,456 were documented from 2007-11. Northern Maine used to be too cold for deer ticks, which transmit Lyme disease through a bacterial infection, but the ticks are now gaining a foothold in more northern counties.