I'm writing a short story that focuses on an area with high thermophile activity, like a hot spring. There are known thermophile viruses, but thermophiles aren't able to transmit these to humans because our bodies are too cold. So we don't know what exactly these viruses could do to a human host if conditions existed for these viruses to be transmitted. I'm struggling to figure out a workaround for this and could use some help. Is there any way that it would be possible somehow to infect a human host with a thermophile virus?
I used to share lab space with people who worked on ancient bacteria (not viruses) including thermophilic ones. They told the tale of their previous lab, where the thermophilic bacteria had 'escaped'*. Some of the bacteria got into the distilled water making machine, which boils tap water and condenses the steam to make distilled water. The bugs took up residence in the boiling vessel and grew colonies there, and no amount of dismantling and cleaning it could shift them. Eventually they gave up trying, and just checked the distilled water for contamination instead. And bought bottles of distilled water from a lab supplies firm when they really, really needed thermophilic bug free water.
The boiling vessel was ideal for the bacteria - nice and hot, with enough nutrients in the tape water for them to live on. They were not bothered by the fact that it got switched off now and then, and cooled down to room temperature.
So where else around the lab is there a nice source of boiling water to live in? The kettle, tea urn, water heater or whatever other gadget provides the scientists or lab techs with hot water to make tea & coffee!
Bacteria (or bacterial spores) from drying mud samples accidentally get transferred from lab bench to lab office kettle. They grow happily in the kettle. And every time you pour yourself a cup of tea, you are drinking some thermophilic bacteria.
Or someone just can't be bothered to walk out of the lab area all the way to the kitchen area to refill the kettle/their water bottle, and refills it from the distilled water reservoir.
If you want viruses instead of bacteria it is the same route, but they won't grow in the kettle/distilled water (since they need a host to reproduce), they'll just sit there. Alternatively the thermophilic viruses are living inside the thermophilic bacteria, because those are their usual host.
*No-one was actually trying to contain them, since they are deep-sea bacteria from ocean floor mud cores, not movie monster bacteria. All the folk in the lab were immunised against the harmful bacteria which are found in shallow sea mud and estuarine mud because that might be contaminated by sewage or farm waste (hepatitis A, typhoid, tetanus and polio, IIRC).
Semi-deliberately? The primary host for these viruses is thermophilic but a research team might extract said viruses from their usual host. They'd need to find a body temperature host to facilitate research into the properties and effects of the virus they wished to study. This new host would necessarily need to be very similar, biochemically speaking, to the original host but live at lower temperatures. The crossover into a human host could then come:
accidentally in the lab.
accidentally when disposal attempts after their experiments prove ineffective, normal medical incinerators may not have what it takes to dispose of these viruses.
accidentally in the course of animal testing.
accidentally due to unexpected contamination by live virus of a product manufactured using the virus.
deliberately in the course of human tests of some beneficial effect discovered in the lab.
An alternative is presented in Peter Watts' Rifters Trilogy wherein heavily altered humans play host to an organism that the human race would not otherwise have ever met and spread it around eventually infecting the whole world.
Since it seems unlikely that thermophiles can infect humans, due to their extremely different environments, you might consider altering your plan slightly.
Thermophile viruses seem to like a temperature range of 55 to 80 degrees centigrade with extremophiles enjoying habitats even above that temperature. Viruses, such as the much loved rhinovirus, like a temperature range of 33 to 35 degrees, a little cooler than core body temperature. Humans, on the other hand, like a very narrow and relatively frigid range of temperatures, 36.5 to 37.5 degrees centigrade. Even raising the core temperature to a relatively moderate 40 to 42 degrees for any length of time will prove deleterious and eventually fatal. (The highest survived temperature I've found is 46 degrees (heat stroke) and the victim was in hospital forever.)
Since there is a very large gap between what the human body likes and what thermophile viruses like (a gap of almost 20 degrees!) you might need to consider infection by some kind of mesothermophile or subthermophile.
A (hypothetical) bug that can (just barely) eke out an existence down to about 37 degrees centigrade might fit the bill. If it can establish itself and trigger its host to react with a fever (38 to 39 degrees), it will be much happier. If it can trigger a raging fever of 40 degrees or higher (survivable by the human for a very short period of time), the virus will be ecstatic!
Therefore, what you might consider is:
a virus that likes a range of temperatures between about 37 and 57 degrees centigrade. I would posit that a hot spring is a good place to look for such bugs, but obviously not in the hottest environments. The humans in your story are obviously interested in thermophiles, but in order to get to the really hot bugs, they would need to traipse through the microenvironments enjoyed by sub-thermophiles, perhaps living along the relatively cooler margins of very hot bodies of water.