Fleas are active from 45 degrees Fahrenheit to about 85 degrees. Optimal is about 70 degrees when used to transmit plague as a bio weapon.
I live at 4600 ft in Pocatello. We never seem to have flea problems. When I lived in San Diego, the flea problem was an issue all year.
Since Denver is at 5,280 feet, I was wondering how fleas could survive and spread bubonic plague at that altitude.