Zika virus has recently emerged in Central and South America, causing millions of cases of fever. This mosquito-borne virus, related to dengue, yellow fever, and West Nile viruses causes a relatively mild bout of flu-like symptoms for a week or so. One notable symptom is reddening of the eyes. In most healthy adults, or even children, the infection is unlikely to cause lasting effects and can be handled with bed rest and pain relievers (in consultation with your doctor, of course). However, there has been an epidemiological link in Brazil between infection with Zika and a rare birth defect, called microcephaly. Babies born with microcephaly are likely to show depressed mental functioning and possibly other physiological problems. Zika is originally from Africa and has circulated in Asia for several years, but the only known association with microcephaly is in Brazil. This suggests that there is either something different about the virus in Brazil or something different about Brazil. Viruses like Zika are highly mutable, and it is possible a new strain of the virus that causes microcephaly evolved. It is also possible that there is something specific in the environment in Brazil that might be linked to the birth defects. Brazil’s robust public health system is investigating, but given the dramatic impact of birth defects, the Centers for Disease Control has issued a travel warning for pregnant women, and several countries affected by Zika had discouraged women from getting pregnant.