The easy answer to that of course is this. You contact your local doctor as soon as possible and schedule a visit. The doctor can then do an emergency diagnosis and bring that fever under control, usually with prescribed medication. But what if this is serious? What if you are bedridden? What if you are physically maimed with the fever-inducing injury, so much so that you cannot even move a muscle?
You can’t even leave the house. Well, in some parts, believe it or not, there is still house calls. And it remains obligatory, particularly if there has been an emergency distress call. But what if you are hit with an injury or suddenly attacked with a shivering or sweating fever and you are quite literally stuck out in the middle of nowhere? This would be a rare occurrence to be sure because there are very few adventurers out there.
And most people, these days, are city dwellers, pretty much close to everything that they need. Any form of inmate fever control could be quickly brought under control if you are properly or adequately prepared. Ironically that would be easier said than done if you were a city dweller. Life is pretty much taken for granted because the sense is there that everything of necessity is always close to hand.
But a sudden fever attack? No one expects that, and hardly ever are they prepared. Now, your adventurous adventurers will probably have thought of all of this, preparing themselves well for the big trip ahead. They would also have taken into account the terrain that they are covering and the surroundings, its peculiarities, the weather, the fauna and even the flora. Snakes can give you the fever. And even rare plants can do that.