Recurrence is the recurrence of a disease after it is believed to be resolved. Classically it appears with conditions such as malaria, in which the parasite responsible for the disease can remain present in the blood in low amounts, reigniting in days, weeks and sometimes months. This term is very similar to "relapse", with some people differentiating between the two by scale; a relapse occurs after a long period of life without symptoms, while a flare appears more quickly.
This return of an infection can sometimes be mistaken for a reinfection. In true recrudescence, the patient usually experiences an infection because the immune system has weakened over time, allowing the infection to recur. In a reinfection, someone picks up an infectious agent again and experiences a new infection. Sometimes it can be difficult to tell the difference in an area where an infection is endemic and it is easy for people to detect new infections.
The recrudescence of malaria is a very common problem, thanks to the fact that the responsible parasites are notoriously difficult to eradicate completely from the body. Over time, they may suppress the immune system, or patients may experience immune suppression as a result of stress, fatigue, or another infection, allowing the parasites to multiply again and contribute to the development of malaria symptoms. The intensity of the recurrence can vary, depending on the patient and the situation.
When a recrudescence occurs, it is treated like the primary infection, with medications given to treat the infection, hopefully eradicating the infectious agent, and treating the symptoms that arise as a result of the infection. With conditions such as malaria, the patient may also be given a prophylactic medication regimen once the outbreak resolves, with the goal of preventing a recurrence in the future. In regions where diseases like malaria are endemic, residents and visitors can also be encouraged to stay on prophylactic drugs, if possible, so they don't get infected in the first place.
People can also refer to "recrudescence" in the sense of any type of return or flogging, even when it is not medical in nature. The word can be used in a way that is linked to unpleasant associations; In other words, the return of the circus might not be called an upsurge because it is seen as a fun event, while the return of militant forces to a war zone might be called so because it is perceived as negative.