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In the Americas, dengue fever intensified as a public health problem during the 1980s -- between 1980 and 1990 more than one million dengue cases were reported. Also, during those years, an increase in cases of the potentially fatal forms of dengue, dengue hemorrhagic fever and dengue shock syndrome, was observed in various countries of the Americas. Dengue is primarily an urban disease of the tropics, and the viruses that cause it are maintained in a cycle that involves humans and Aedes aegypti. One of four closely related, but antigenically distinct, virus serotypes, DEN-1, DEN-2, DEN-3, and DEN-4, of the genus Flavivirus, cause dengue and dengue hemorrhagic fever. Primary infection with any serotype may lead to acute illness defined as fever with two or more of the following symptoms: headache, bodily pain, rash, and hemorrhagic manifestations. Fever and other symptoms may subside after 3 or 4 days, and the patient may recover completely, or the fever may return with a rash within 1 to 3 days.  Moreover, infection with one of these serotypes does not provide cross-protective immunity, so persons living in a dengue-endemic area can have four dengue infections during their lifetimes. Infection with a dengue virus serotype can produce a spectrum of clinical illness, ranging from a nonspecific viral syndrome to severe and fatal hemorrhagic disease. Secondary exposure to the same serotype generally does not produce illness because of pre-existing antibodies. However, secondary exposure to a different serotype may lead to another dengue fever episode, and the patient may be at risk for more serious forms of infection, dengue hemorrhagic fever or dengue shock syndrome. Furthermore, dengue virus infection may also cause a nonspecific febrile illness that can be easily confused with measles or influenza. Therefore, a proper laboratory test is essential for a correct differential diagnosis and to public health surveillance reporting.