Tom Frieden is director of the U.S. Centers for Disease Control and Prevention.

(CNN) Vaccines and antibiotics have made many infectious diseases a thing of the past; we've come to expect that public health and modern science can conquer all microbes. But nature is a formidable adversary. And Zika is our newest threat, particularly to pregnant women.

New, unfamiliar and mysterious threats to our health are scary. At the Centers for Disease Control and Prevention -- where we identify, on average, one new health threat each year -- we work around the clock with an approach that prioritizes finding out what we need to know as fast as we can to protect Americans.

The CDC has some of the world's leading experts both in diseases spread by mosquitoes and in fetal abnormalities. We get the facts, base actions on science, tell people what we know when we know it and what we are doing to add to our knowledge, and act to protect Americans today as effectively as possible.

Who is at risk for Zika infection?

Most people in the contiguous United States are unlikely to ever come into contact with the Zika virus, but two groups need particular attention. First, people living in the Commonwealth of Puerto Rico, a U.S. territory, the U.S. Virgin Islands, Caribbean or Pacific territories, and Central and South America are likely to see an increasing spread of Zika. In these areas, women who are pregnant need to protect themselves from mosquito bites by using repellants, permethrin-coated clothing, long sleeves and pants, and by staying indoors (ideally in places with air conditioning) as much as is practical. We advise pregnant women to postpone travel to areas where Zika is spreading.

Photos: Zika virus outbreak Photos: Zika virus outbreak A pest control worker fumigates a school corridor on the eve of the annual national Primary School Evaluation Test in Kuala Lumpur on Sunday, September 4. Malaysia reported its first locally transmitted Zika case on September 3. Hide Caption 1 of 29 Photos: Zika virus outbreak A banner about Zika virus is seen as ferry passengers arriving from Singapore get in line at the immigration check on September 4, in Batam, Indonesia. Hide Caption 2 of 29 Photos: Zika virus outbreak A banner is flown over the South Pointe Park area, Tuesday, September 6, in Miami Beach, Florida. Hide Caption 3 of 29 Photos: Zika virus outbreak A pest control worker fumigates drains at a local housing estate where the latest case of Zika infections were reported on Thursday, September 1, in Singapore. Hide Caption 4 of 29 Photos: Zika virus outbreak Florida Gov. Rick Scott and Florida Surgeon General Dr. Celeste Philip address the media gathered at the Miami-Dade County Department of Health as they announce five cases of Zika in a 1.5 mile area of Miami Beach on Friday, August 19, in Miami, Florida. Hide Caption 5 of 29 Photos: Zika virus outbreak Brazil's interim President Michel Temer, center right, meets with officials during Temer's first visit to the Olympic Park on Thursday, June 14, in Rio de Janeiro. The Rio 2016 Olympic Games commence August 5 amid a political and economic crisis in the country along with the Zika virus outbreak. Hide Caption 6 of 29 Photos: Zika virus outbreak Rio 2016 Chief Medical Officer Joao Grangeiro, Municipal Secretary of Health Daniel Soranz and Sub-secretary of the State for Health and Surveillence Alexandre Chieppe field questions from the media during an International Media Briefing to address the Zika virus on Tuesday, June 7, in Rio de Janeiro. Hide Caption 7 of 29 Photos: Zika virus outbreak A health worker fumigates an area in Gama, Brazil, to combat the Aedes aegypti mosquito on Wednesday, February 17. The mosquito carries the Zika virus, which has suspected links to birth defects in newborn children. The World Health Organization expects the Zika outbreak to spread to almost every country in the Americas. Hide Caption 8 of 29 Photos: Zika virus outbreak A man places a mosquito net over a bed at a home for the elderly in Masaya, Nicaragua, on Thursday, February 11. Hide Caption 9 of 29 Photos: Zika virus outbreak An Aedes aegypti mosquito floats in stagnant water inside a tire at a used tire store in Villavicencio, Colombia, on Thursday, February 4. Hide Caption 10 of 29 Photos: Zika virus outbreak A health worker fumigates an area in Caracas, Venezuela, on Tuesday, February 2. Hide Caption 11 of 29 Photos: Zika virus outbreak A lab worker exposes his arm to Aedes aegypti mosquitoes during testing at the Roosevelt Hospital in Guatemala City, Guatemala, on Monday, February 1. Hide Caption 12 of 29 Photos: Zika virus outbreak Tainara Lourenco sits inside her home in Recife, Brazil, on Friday, January 29. Lourenco, five months pregnant, lives at the epicenter of Brazil's Zika outbreak. The Zika virus has been linked to microcephaly, a neurological disorder that results in newborns with small heads and abnormal brain development. Hide Caption 13 of 29 Photos: Zika virus outbreak Dr. Angela Rocha shows brain scans of a baby born with microcephaly at the Oswaldo Cruz Hospital in Recife on Thursday, January 28. Hide Caption 14 of 29 Photos: Zika virus outbreak Alice Vitoria Gomes Bezerra, a 3-month-old baby with microcephaly, is placed in her crib by her father Wednesday, January 27, in Recife. Hide Caption 15 of 29 Photos: Zika virus outbreak A health ministry employee fumigates a home in Soyapango, El Salvador, on January 27. Hide Caption 16 of 29 Photos: Zika virus outbreak A lab technician at the Oswaldo Cruz Foundation Institute stores Aedes aegypti mosquitoes to be used in research in Recife on January 27. Hide Caption 17 of 29 Photos: Zika virus outbreak A patient suffering from Guillain-Barre syndrome recovers at a hospital in San Salvador, El Salvador, on January 27. Researchers are looking into a possible link between Zika and Guillain-Barre, a rare disorder that causes the body's immune system to attack its nerves. Hide Caption 18 of 29 Photos: Zika virus outbreak Luiz Felipe lives in Recife and is one of more than 4,000 babies in Brazil born with microcephaly since October. The drought-stricken impoverished state of Pernambuco has been the hardest-hit, registering 33% of recent cases. Hide Caption 19 of 29 Photos: Zika virus outbreak A health worker sprays insecticide under the bleachers of Rio de Janeiro's Sambadrome on Tuesday, January 26. Hide Caption 20 of 29 Photos: Zika virus outbreak A Brazilian soldier inspects a home in Recife on Monday, January 25, while canvassing the neighborhood and attempting to eradicate the larvae of mosquitoes linked to the virus. Hide Caption 21 of 29 Photos: Zika virus outbreak David Henrique Ferreira, a 5-month-old who has microcephaly, is watched by his brother in Recife on January 25. Hide Caption 22 of 29 Photos: Zika virus outbreak The larvae of Aedes aegypti mosquitoes are photographed in a lab in Cali, Colombia, on January 25. Scientists are studying the mosquitoes to control their reproduction and resistance to insecticides. Hide Caption 23 of 29 Photos: Zika virus outbreak Angelica Prato, a pregnant woman infected by the Zika virus, receives medical attention at a hospital in Cucuta, Colombia, on January 25. Hide Caption 24 of 29 Photos: Zika virus outbreak A woman walks through fumes as health ministry employees fumigate an area in Soyapango on Thursday, January 21. Hide Caption 25 of 29 Photos: Zika virus outbreak Brazilian soldiers apply insect repellent as they prepare for a cleanup operation in Sao Paulo on Wednesday, January 20. Hide Caption 26 of 29 Photos: Zika virus outbreak A researcher at the University of Sao Paulo holds a container with female Aedes aegypti mosquitoes on Monday, January 18. Hide Caption 27 of 29 Photos: Zika virus outbreak A graveyard in Lima, Peru, is fumigated on Friday, January 15. Hide Caption 28 of 29 Photos: Zika virus outbreak Aedes aegypti mosquitos are seen at the University of Sao Paulo on January 8. Researchers from the Pasteur Institute in Dakar, Senegal, came to Brazil to train local researchers to combat the Zika virus epidemic. Hide Caption 29 of 29

The spread of the virus through blood transfusion and sexual contact have been reported in isolated cases. However, for most of the nonpregnant population, there is no reason to think Zika presents a particular risk.

Will we see Zika in the U.S.?

We have already seen the Zika virus in travelers returning from places where Zika is spreading, including, sadly, one woman in Hawaii who delivered an infant with microcephaly after being infected with the virus in Brazil last year. We will certainly see more travelers returning to the United States with Zika after being infected in parts of the world where the virus is spreading. But the big question many people have is whether Zika will spread widely within the United States.

Science doesn't have a crystal ball, but the CDC has great laboratories and the world's best disease detectives. For a disease such as Zika to spread widely, two things are necessary. The first is the specific mosquito species that spreads the virus. The second is the conditions in communities; places that are crowded and don't have air conditioning enable viruses such as Zika to spread.

So we do expect, unfortunately, that Puerto Rico and the U.S. Virgin Islands could have many infections with the Zika virus, and we will certainly see U.S. travelers returning with Zika infections, just as we saw travelers returning with dengue and chikungunya infections. We could see isolated cases and small clusters of infections in other parts of the country where the mosquito is present. But from the information we know now, widespread transmission in the contiguous United States appears to be unlikely.

What is the government doing?

Since the first large Zika outbreak ever recognized, in 2007, the CDC has had boots on the ground responding. Our laboratories have developed a test that can confirm Zika in the first week of illness or in a sample from an affected child. Diagnosing prior infection with Zika is much more challenging, and CDC scientists as well as private companies are working to develop tests that can do this accurately. This is a priority, and we are working to do in weeks what would usually take months or years.

We are supporting laboratories in Puerto Rico and around the United States to provide testing, and we are using cutting-edge genomic methods in this effort. We are also working with Puerto Rico and other places at risk around the country to improve mosquito control efforts before we head into warmer weather when mosquitoes become a bigger problem. The CDC also provides support and guidance for health care providers and the public. You'll know of any new developments as soon as we do.

Across the Department of Health and Human Services, there is also important work related to Zika , particularly to speed the development of tests, treatments and vaccines

Prevention will be key. Mosquito control is hard. States and cities that invest in mosquito control can track and fix many places where mosquitoes can breed to drive down mosquito populations. But this takes hard, meticulous work -- and money. We must maximize the use of today's tools to reduce the mosquitoes that can spread Zika and other diseases. We must also advance innovative mosquito control tools of tomorrow, such as promising new products that may be safer and more effective than today's methods.

There is no way to predict when or where health threats will emerge, but the plain fact is that we will continue to see new infectious disease threats such as Zika. The CDC's laser focus is protecting the health, safety and security of Americans; learning more about Zika and fighting it is a top priority.