Pyrexia of unknown origin (PUO)
Sometimes, fever can have no detectable cause either by clinical examination or laboratory investigations, and in such case, it is called a pyrexia of unknown origin. For fever to be named as PUO, it has to last for more than 3 weeks and to be more than 100.9 °F. Being of an unknown origin doesn’t mean that it doesn’t have one, however, and further, more detailed investigations would be needed.