The World Health Organization has declared an international health emergency over the spread of the Zika virus, now known to cause devastating birth defects. The agency expects the virus to spread from northern Argentina to the southern United States by the end of the year, infecting many millions of people. In late July, Florida officials announced what appeared to be the first locally transmitted cases of Zika infection in the continental United States.

The W.H.O. and the Centers for Disease Control and Prevention have urged pregnant women against travel to more than 45 countries in which the Zika virus is spreading, mostly in the Caribbean and Latin America. All pregnant women who have been to these regions should be tested for the infection, health officials have said, and should refrain from unprotected sex with partners who have visited these regions.

The Zika virus has been linked to unusually small heads and brain damage in newborns — called microcephaly — in children born to infected mothers, as well as blindness, deafness, seizures and other congenital defects. In adults, the virus is linked to a form of temporary paralysis, called Guillain-Barré syndrome.

Here are some answers and advice about the outbreak.