Fever and weakness are typically caused by an infection or illness in the body. Common illnesses that lead to fever and weakness include the flu and chicken pox, among others. Viral or bacterial illnesses can cause a fever and lead one to feel weak; in addition, any infection, even a small one, can be a cause of a fever. This means that a low-grade fever can be indicative of an infection, even if one is unaware of an infection in the body. Because these symptoms are so common, a doctor may need to do a blood test or throat swab to determine what type of illness is present.
In most cases, there is no treatment for a viral illness that causes fever and weakness. Antibiotics are ineffective against viral illnesses, though it may be a good idea to visit a doctor anyway to be sure that the cause of the fever is not anything serious. Generally, a virus will cause a lower grade fever than a bacterial infection, but this is certainly not always the case. The fever should not last more than a few days, and usually getting some rest and drinking plenty of fluids is an effective treatment for mild illnesses.
A bacterial infection that causes fever and weakness can usually be treated by antibiotics. Again, virtually any infection can cause a fever; it is also possible for any infection to spread quickly, so it is necessary to keep a close eye on it, and visit a doctor if it worsens. If one has an infection from a cut on the skin, for example, and is experiencing fever and weakness, it is a good idea to visit a doctor. In addition, any skin infection with red streaks coming off it has entered the bloodstream, which can cause septic shock and death; one should immediately get emergency treatment and intravenous antibiotics.
Experts are occasionally divided as to whether medication should be taken to lower a fever. If the fever is high, or the person with the fever is elderly or very young, medication should be given to reduce the fever. On the other hand, if it is a relatively low-grade fever and the person is otherwise in good health, it may be best to allow the fever to run its course; the fever is the body's way of fighting off an infection, and it can actually be helpful. Fevers naturally cause fatigue and weakness, so if it is possible for one to just get some rest until one recovers, that can often be the best method of treatment.