If you’re thinking about donating blood and have traveled recently to areas affected by the Zika virus, federal officials are saying hold off. There have been no reports so far of the virus getting into the U.S. blood supply, the Food and Drug Administration said. But out of an abundance of caution, the FDA is asking travelers to postpone any planned donations for at least four weeks. There is a risk contamination could happen, given the way the virus spreads and the fact that transfusions have led to Zika transmissions in countries outside the United States. Read: Zika explained It’s tricky The FDA guidelines might seem strict to some, but keeping Zika completely out of the blood supply could be tricky. Four out of five people who contract it never show symptoms, and as a result, 80% of transmissions are never diagnosed, says the Centers for Disease Control and Prevention. So, a blood donor carrying the virus can feel just fine, think nothing is wrong and end up unwittingly passing on the virus when they give blood. Read: How to stop Zika’s spread Caution period Anyone who has had a confirmed infection, possibly been exposed to the Zika virus or traveled to areas where the virus is being transmitted, should postpone donating blood for the four week period, the FDA said. Also, people who have had sexual contact with someone who has been to an area where the virus is actively being transmitted should also wait as long to donate. The virus is currently actively being transmitted in mostly tropical and subtropical areas, particularly in South and Central America, central Africa, and South East Asia, the CDC says. The four-week wait should provide plenty of time for an infected person’s immune system to clear the virus out. The CDC has said the virus appears to be out of the blood stream of infected people in about a week. Blood donations should not be collected in U.S. territories where Zika is being transmitted. The Zika scare Concern about Zika erupted in recent months, although the virus was first identified in 1947, according to the World Health Organization. In the past, Zika generated less fear because if an infected person developed symptoms, they were usually mildly flu-like, run their course then disappear. But then last year, researchers began seeing cases of a usually rare but cruel birth defect balloon in tandem with a Zika outbreak in Brazil. Thousands of babies of mothers who had been infected during pregnancy were born with heads markedly smaller than normal, combined with incompletely formed brains, a condition called microcephaly. Scientists have not yet been able to confirm that Zika causes the microcephaly, but the parallel surges in both have accelerated research and prompted public health officials to make recommendations as though they are directly connected. Blood-to-blood Zika is almost exclusively transmitted blood-to-blood, which means a transfusion from an infected blood donor could lead to an infection. But usually mosquitos are the culprits, biting an infected person then subsequently biting a victim and injecting the virus. Though dozens of cases have been reported in the continental United States, infected patients are believed to have contracted the virus while traveling to affected areas. A handful of direct infections have been noted in the U.S. territories Puerto Rico, the Virgin Islands and American Samoa. Previous steps The country’s largest blood collection organization, the American Red Cross, had already issued donation guidelines to protect against Zika entering its blood supply. It, too, asked donors who had traveled to affected areas to wait four weeks before donating. The Canadian Blood Services issued similar guidelines a week prior.