Infection caused by the Western Nile virus, which is an arbovirus-flavivirus transmitted by mosquito bite.Reservoir: birds. Vector: mosquitoes. Host: horses and humans.The disease usually appears asymptomatic (80% of cases) or with mild flu symptoms. Less than 1% of affected patients develop meningoencephalitis and other serious features. It has an incubation period from 5 to 15 days. Although there has been a documented case of transmission through breastfeeding (positive RNA and specific IgM antibodies in the milk with positive plasma IgM antibody in the infant), the child remained asymptomatic (CDC 2002). Among six breastfed infants whose mothers were diagnosed as positively infected, who subsequently had serologic tests for West Nile virus that were negative, only one of them exhibited a slight skin rash (Hinckley 2007). Under current information, the benefits of breastfeeding outweigh the risk of disease in infants, in such a way that mothers, even from endemic areas, should be encouraged to breastfeed their infants (Hayes 2005).