A viral illness, such as measles, rubella, chickenpox (varicella), or mumps.
Mononucleosis (Epstein-Barr virus), which results in fever, sore throat, and fatigue, or cytomegalovirus (CMV), a viral infection that causes symptoms similar to those of mononucleosis.
A bacterial illness, such as strep throat (caused by the streptococcus bacterium) or Lyme disease (a bacterial infection spread by certain types of ticks).