US President Barack Obama has called for urgent action against the Zika virus, which has been linked to babies being born with underdeveloped brains.
He said research to develop vaccines and treatments must be speeded up.
The World Health Organization says the virus is likely to spread across nearly all of the Americas.
It has already been found in 21 countries in the Caribbean, North and South America. Symptoms include mild fever, conjunctivitis and headache.
Medics are most worried about the damage the virus can potentially cause to babies in the womb.
Brazil has recorded a huge spike in cases of microcephaly - the condition where babies are born with unusually small heads - and a number of Central and South American nations have asked women to delay pregnancy.
There have been 3,893 reported cases of microcephaly in Brazil since October compared with the previous annual average of just 160 cases.
- A Danish tourist returning home from Latin America has been diagnosed with the virus. Earlier cases in Europe include Germany, Britain and Sweden
- Health officials in the US states of Virginia and Arkansas say two residents who travelled outside America have tested positive for the virus, the AP news agency reported
- Latin America's largest airline, Grupo LATAM, has announced it is waiving cancellation or flight-change fees for pregnant women wanting to cancel flights to affected countries