If he catches any form of sickness it becomes a fever almost without fail, if it is anything more than a cold or a headache.His fever normally breaks after two days.The severity varies depending on what he actually came down with. Sometimes he’ll just be running unhealthily hot, but power through it. Other times, he’ll be unable to lift his head from his pillow without feeling lightheaded.
The few times when Tyson can remember avoiding a fever when getting sick was when he has food poisoning and chicken pox. The first fever Tyson is aware of having was getting hypothermia after going swimming with Jethro in Autumn.
This happens for several reasons. Tyson chronically under-eats, despite pushing to keep himself to the same physical standard that he’d been in the army and the extra food his body needs to heal from his various injuries working for the web. His apartment, due to poor insulation in the walls and a temperamental boiler, is often cold as ice.
In addition to that, Tyson’s sleep schedule is messed up in a dozen different ways. He lets the Moriarty Mirrors use him as a Guinea pig in their experiments, including those around chemicals and nanobots, and occasionally dabbles in drug-painkillers: sometimes illegal ones, and more illegal ones from the Moriarty Mirrors experiments.
All of those things add up to weaken Tyson’s immune system, because he’s wearing himself thin without reprieve. As such, when he gets infected - which is relatively easy to due in a close quarters, high population city with people constantly entering from around the world like London - Tyson’s body can’t put up that much of a fight.
Natural genetic luck, and staying on top of his shots, means that Tyson has a pretty strong immunity towards getting infected in the first place, but once he becomes ill, his body goes into completely shut down and purge mode. This also means that after his fever passes, Tyson generally doesn’t have a long tail on recovering as his body burns off the sickness.