Are disease causing viruses more active in the heat?

Not necessarily: Viruses aren't really more or less active based on the temperature. However, being overly tired from working in the heat can affect your immune system and make you more susceptible to getting sick from a virus. That's one reason why it's important to stay well-hydrated and well-rested when working in the heat.

Heat,cold: Some viruses thrive in the heat, but flu is more prevalent in cold weather. However in extremely cold weather people don't get out and about as much and thus there is less spreading of the viral infections.