Common causes of unexplained fever include bacterial (brucellosis, mycoplasma, bartonellosis, etc.), fungal (histoplasmosis, coccidioidomycosis, etc.), rickettsia (Ehrlichia) body diseases, etc.), viral (canine distemper, etc.), protozoal (babesia, pyrozoia, leishmaniasis), immune-mediated (meningitis, etc.), neoplastic and others.