The optimum temperature and environment for fleas is between 70°F and 85°F. They also require relatively high level of moisture and humidity in the air, with a preferred level of about 70% humidity.
It is difficult to get rid of a flea infestation by subjecting it to heat or cold, however, because fleas are very hardy insects and their pupa can survive harsh conditions only to emerge later and become active adults.
A pupa is the stage in the life cycle of a flea in which it hibernates and goes inactive. It forms a sort of cocoon around itself during this time.
Normally a flea will only spend about 1 to 2 weeks in this stage, but if conditions are harsh or a suitable host is not present, it can extend this period for months or even years until it detects more optimal conditions.
A cold and harsh winter can lower or limit the activity of a flea infestation, however, it will most likely not get rid of it entirely. Fleas are most active during the heat of the summer in most areas.
Washing clothes and linen in hot water and detergent does a good job at killing fleas, and so does putting it in the clothes dryer.
But the most effective way to deal with fleas is to use a suitable flea medication for your pets.