My go to for whenever I'm feeling ill has been to hop into bed bundled up to the nine and have a long sleep. I'll usually wake up with an unnatural amount of sweat, to the point that sheets need to be changed. I'd start feeling better that day or the next. I could be wrong but there's probably a connection between where people tied cold weather to illness. "Bundle up or you'll catch a cold". It's not that illness came from the weather, it's that the body temp can get so low that it's more susceptible to infection.