Infections in the elderly, similar to other acute illnesses in this age group, may present in atypical, nonclassical fashions. Fever, the cardinal sign of infection, may be absent or blunted 20%-30% of the time. An absent or blunted fever response may in turn contribute to diagnostic delays in this population, which is already at risk for increased morbidity and mortality due to infection. On the other hand, the presence of a fever in the geriatric patient is more likely to be associated with a serious viral or bacterial infection than is fever in a younger patient. Finally, a diagnosis can be made in the majority of cases of fever of unknown origin (FUO) in the elderly. FUO is often associated with treatable conditions in this age group.