The World Health Organization on Monday declared a global public health emergency over the Zika virus, which it said is “spreading explosively” in South America. The announcement came after an emergency session in Geneva.

In addition, a second case of the virus has been reported in Virginia. Officials at the College of William & Mary said a student who visited Central America during winter break contracted the illness, but they do not believe this poses a health risk to anyone on campus.

The virus, which first emerged in Uganda’s Zika Forest after World War II, appeared in Brazil last May and has been linked to thousands of cases of microcephaly, a condition in which infants are born with small heads because of poor brain development during their mother’s pregnancies.

Dr. William Petri, Wade Hampton Frost Professor of Medicine in the University of Virginia’s School of Medicine and chief of the Division of Infectious Diseases and International Health, spoke with UVA Today about the spread of the Zika virus and its implications.

Q. Can you describe the Zika Virus? How is it spread?

Infectious diseases expert Dr. William Petri said an American outbreak of Zika is not imminent. (Photo by Sanjay Suchak)

A. The virus can cause a fever, rash and joint aches and you can also get pink eye. The Zika virus until last year had mostly been seen in parts of Africa and Asia. What’s new about Zika virus is its appearance in Brazil in May and then the increased number of cases in South and North America.

Q. How is it spread?

A. Zika virus is spread by mosquito bites. It is in the same family of viruses as dengue and yellow fever. They are all spread by mosquitoes. It’s in the family of Aedes mosquitoes. They exist everywhere from Argentina to the southern United States.

Q. Why did it suddenly appear in Latin America?

A. We probably will never know why it appeared in Brazil. Presumably what happened was a traveler from Africa or Asia who had the virus in their bloodstream was bitten by a mosquito in Brazil, setting up a chain of endemic transmission of Zika virus.

Q. The illness has been linked to very serious birth defects. What do you know about how Zika can affect newborns?

A. I don’t think we know yet. There is a concerning association of microcephaly, or small heads, in children born of mothers who were infected with Zika virus. This has been seen in Brazil. But right now it is an association and it has not been conclusively shown that it is due to Zika virus.

Q. There are 31 reported cases of Zika virus in the United States, one of those in Virginia. Is the United States at risk of an outbreak of the virus?

A. Fortunately, today in the United States we are not having transmission of Zika virus. The cases of Zika infection that we have had are all in travelers who are returning from countries where there is transmission.

Virginians should not be worried right now because we are in the midst of winter and the mosquitoes are all dead.

Q. The WHO says as many as 4 million people could be infected by the end of the year. Why is Zika virus spreading so rapidly?

A. It’s probably a couple of things. Mosquito-borne illnesses have the capability of rapid spread. The other thing is that there is not preexisting immunity at all to Zika. This is a new viral infection in the Americas. The WHO projections are probably pretty reasonable as far as the number of cases to expect.

Q. What precautions can people, particularly women of childbearing age, take to protect themselves?

A. There is enough of a concern that it is prudent that someone who is pregnant not travel to an area where there is Zika virus transmission. The use of mosquito repellent is really very important for all travelers, not just women who are pregnant, because that will not only help prevent Zika virus, but dengue, malaria, leishmaniosis or other vector-borne diseases.

It is also important from a public health standpoint that travelers to Zika endemic areas use mosquito repellant, because what we don’t want to do is establish endemic transmission in the United States. If we had a number of travelers coming back to the United States with Zika virus in their bloodstream, then we have the possibility of actually establishing transmission with the Aedes mosquitoes that are in the southern U.S.

Q. There is no vaccine for Zika virus, which is in the same family as yellow fever. UVA medical school graduate Walter Reed showed that yellow fever is transmitted by mosquitoes, a discovery that led to the creation of a vaccine for the virus. Could this work have an impact on creating a vaccine for Zika virus?

A. You could say that Walter Reed laid the groundwork in 1900 for the discovery that Zika virus is mosquito-borne.

Q. Brazil is hosting the Summer Olympics. How can Rio make the environment safe for this huge event?

A. I don’t know, but I would assume Brazil will have a very, very aggressive control program for mosquitoes in the areas surrounding the Summer Games. That would be predicted to be very effective.

The reason the Panama Canal was successfully built by the Americans was Walter Reed’s discovery that yellow fever was transmitted by mosquitoes. Then-Surgeon General William Gorgas instituted a mosquito control program in Panama that allowed them to successfully build the canal without all the workers contracting yellow fever.

Q. Can people be re-infected by Zika virus?

A. No. It is a self-limited infection. You recover from it and are immune to re-infection. So this explosive case of infections cannot continue. As immunity in the population grows, the transmission will slow. Mosquito control programs will go a long way and the use of personal insect repellant like DEET will help protect the individual.