A C.D.C. epidemiologist, Dr. Erin Staples, said earlier this month that she expected Zika to follow the same pattern as other mosquito-borne viruses like dengue and chikungunya: large outbreaks in Puerto Rico followed by smaller ones in Florida, Gulf Coast states and possibly Hawaii.

A study published online by The Lancet on Friday reached similar conclusions after analyzing mosquito species ranges and air-travel patterns from Brazil.

Only one case has been confirmed in Puerto Rico, but because testing is rare and many cases show no symptoms, doctors assume there are many more. How far it spreads, officials said, will depend on which mosquitos prove adept at transmitting it and how aggressive mosquito control efforts are. The yellow fever mosquito, Aedes aegypti, is common in Florida and the Gulf Coast and is an efficient Zika transmitter. The Asian tiger mosquito, Aedes albopictus, ranges as far north as New York but it is not yet known whether it transmits Zika.

The virus first appeared on the South American continent in May. Although only one person in five ever gets symptoms, and even then it often causes only mild rashes, red eyes and fevers, women who have had it, particularly in the first trimester of pregnancy, appear to be much more likely to have children with small heads and damaged brains, a condition called microcephaly.

Zika virus has been found in brain tissue and amniotic fluid from babies who died in the womb or were born with microcephaly by both Brazilian and American scientists. Microcephaly has several other causes, including genetic defects, alcohol exposure in pregnancy, or rubella or cytomegalovirus in the mother during pregnancy.

Scientists do not know why or how Zika crosses the placenta and enters the fetal brain to do damage. It is not related to rubella or cytomegalovirus, but is related to yellow fever, dengue and West Nile virus, which are not widely known to harm embryos.