Yep, you'd want to put it on a timer. Optimally trigger it off some natural event, maybe temperature, so it can achieve universal penetration before activating.

Sounds a little bit sci-fi I don't think there is any viruses that exist that are just triggered off temperature ?

I mean I don't know, I'm not a biologist. But existing viruses are not optimized for sneaky mass deaths.

If one doesn't exist, how do you know one can ?

Well, it's rather more "if you're not a biologist, how do you know it can exist?" Things that don't exist yet are often very predictable. However, it simply seems to me that "Viruses can not condition on environmental temperatures or the side effects of environmental temperature in the body, such as response to UV exposure" is a quite implausible statement, and I have not heard of anything of the sort.