The emergence of the Zika virus and the health crisis it has ignited has frightened American citizens traveling abroad—and for good reason. Zika virus can cause birth defects to babies infected in utero, including potentially lifelong damage to the nervous system. What most American travelers don’t realize, however, is that there is another virus out there that is far more common, far more contagious, and far more devastating to the unborn child. Most people just thought it was gone.

The Zika story is relatively recent. In 1947, a new virus was isolated from a rhesus monkey in the Zika Forest of Uganda. During the next six decades, Zika virus caused disease in only 13 people.

In 2007, Zika surprised public health officials when it infected 5,000 people—about three quarters of the population—on the island of Yap, part of the Federated States of Micronesia. Between 2013 and 2016, Zika virus caused several more outbreaks in the Pacific island nations of French Polynesia, New Caledonia, Easter Island, Samoa, American Samoa, and the Cook Islands. Because Zika infections appeared to be benign, these outbreaks attracted little attention. About 80 percent of those infected had no symptoms. The remaining 20 percent suffered fever, pink eye, joint pain, headache, rash, and muscle pain—relatively mild symptoms that were short lived and without consequence.

In March 2015, Zika entered the Americas, causing an outbreak in Brazil. By December, the Brazilian Ministry estimated that up to 1.3 million people had been infected. During the Brazilian outbreak, doctors also discovered something else—a dramatic increase in the number of children born with microcephaly (small heads). By February 2016, when it became clear that Zika virus could cross the placenta and infect the unborn child, more than 4,300 cases of microcephaly had been recorded.

In 2016, Zika virus spread into the United States; in Florida, about 200 people were infected and in Texas, six. The U.S. experience also taught public health officials that men infected with Zika virus could transmit the virus to women during sexual intercourse and that the virus could be detected in sperm for up to two months after infection.

Currently, a vaccine to prevent Zika virus isn’t commercially available. If it were, I suspect that most people in the United States planning on traveling to a country where Zika virus has been circulating would get it. At the same time, most American travelers wouldn’t give a second thought to a virus that is arguably far more worrisome: rubella.

The differences between Zika and rubella are striking.

Zika virus has been reported in 60 countries, rubella virus in 129. Last year more than 100,000 cases of rubella occurred worldwide; given that underreporting is common, that figure is probably much higher.

Zika virus infects the brain of the unborn child. Rubella virus infects every organ, causing deafness, blindness, cataracts, retinitis, glaucoma, encephalitis, severe heart abnormalities, severe intrauterine growth retardation, bone thinning, enlargement of the liver and spleen, bleeding, pneumonia, diabetes, hypothyroidism, and autism.

Between 1 to 13 percent of women infected with Zika virus in the first trimester of pregnancy will deliver babies with birth defects. For rubella virus, first trimester infections carry a risk of birth defects of 90 percent.

During the Zika outbreak in Brazil, about 4,000 children suffered microcephaly. During a rubella outbreak in the United States between 1963 and 1964—five years before a vaccine was available to prevent it—rubella infected 12.5 million people causing more than 6,000 spontaneous abortions, 5,000 therapeutic abortions, and 20,000 cases of severe, permanent birth defects. During this outbreak, 1 of every 100 births in Philadelphia was complicated by congenital rubella infection.

Zika virus is spread primarily by the bite of the Aedes aegyptii mosquito, which doesn’t exist in many countries and regions. Rubella virus is far more contagious, spread by sneezing, coughing, or talking.

The contrast between Zika and rubella viruses offers several ironies. First, when people of childbearing age travel overseas, they may avoid a country where Zika virus is circulating but enter a country where rubella virus is common, which would be risky if they haven’t been vaccinated or previously infected with rubella.

Recent outbreaks of measles in the United States have centered on children who never received the measles-mumps-rubella (MMR) vaccine, begging the question of whether outbreaks of rubella might not be far behind. Also, what will happen to these unvaccinated children when they grow up and travel overseas? Natural rubella infection during pregnancy increases the risk of autism in the unborn child. Therefore, parents who have chosen not to give their children MMR vaccine for fear that it causes autism—a fear that has been refuted by 17 studies—are increasing their risk of having a grandchild with autism.

Sadly, as is so often the case, we don’t know where the greater risks lie.