Wear a condom: That has been the standard—and strong—advice from public health officials trying to thwart the spread of HIV or syphilis. The U.S. Centers for Disease Control and Prevention has spent decades trying to get people to put them on. But now health workers are pushing the latex prophylactic for a different reason: Ebola recovery.



People are surviving the disease. Doctors Without Borders, which oversees many Ebola clinics in west Africa, is sending home recovered Ebola patients with a stack of condoms, and health workers are urging them to only engage in protected sex for at least three months after recovery. The virus has been found in the semen and vaginal fluids of convalescents for weeks or even months after symptoms of Ebola have abated, setting off concern that the virus could be spread via sexual contact with otherwise healthy individuals. In men, one study found that Ebola continued to persist in semen for 90 days. U.S. health officials are echoing this caution as a small number of patients have been released from American hospitals.



To date, however, there has not been a single documented case of Ebola transmission from sexual activity. Moreover, simply detecting the genetic presence of the virus in recovering patients does not automatically mean that disease transmission could or would take place—especially if the virus is only present in relatively low concentrations. Although a whole, functioning virus is needed to transmit an infection to another person, current testing methods are also so sensitive they also detect nucleic acids from the virus that continue to lurk in bodily fluids during recovery. “It’s essentially like finding a bone of an animal but that doesn’t tell you if there’s a live breathing animal,” says Daniel Bausch, a professor of tropical medicine at the Tulane School of Public Health and Tropical Medicine.



That may be why one 1999 study in the Democratic Republic of the Congo, which followed 29 people recovering from Ebola and their household contacts (including sex partners) for up to 21 months, found that although four of the five tested convalescents had at least one semen sample with detected Ebola virus inside it none of their sexual partners developed symptoms of Ebola, even if they had unprotected sex during that period.



So why the “safe sex” warning when thousands of patients have survived Ebola and may have gone on to have sex, apparently without infecting their partners? Extreme caution is not an overreaction with this disease. Studies by Bausch and others have also detected live Ebola virus in sexual fluids that can successfully grow in cell culture, suggesting it could also lead to infections in other individuals. It is possible that sexually transmitted Ebola may have flown under the radar because there has been a dearth of data from outbreaks in years past. Also, although extremely unlikely, it is possible that mild Ebola—with very minor symptoms that were not recognized as such—has developed in patients’ sexual partners. Thus, the CDC warns that convalescing patients must either abstain from intercourse and oral sex for three months or use condoms for that entire time.



With any infectious disease, when patients have a high viral load in their bodily fluids, it increases the risk they will pass disease to someone else through direct contact with those fluids. With HIV, for example, the risk of passing the disease between partners increases with higher viral load: For every 10-fold increase in viral concentration, one 2012 study suggests there is about a threefold increase in the risk of transmission per sexual act. And with HIV, condoms are a highly effective mode of blocking disease transmission because the virus is primarily spread via contact with sexual fluids or blood.



As with HIV, when Ebola progresses, a patient’s viral loads inch upward and that boosts the chance of disease transmission via contact with bodily fluids. Moreover, a certain degree of natural immunological protection for certain body parts—the central nervous system, eyes and gonads—makes it difficult for virus to exit those bodily parts, which may lead to the virus continuing to be present even after the virus was cleared from the blood, according to Bausch. And if an Ebola patient’s disease proves fatal, his viral load at death is particularly high, which boosts the risk of contracting the disease from interacting with the corpse.



Ebola virus manages to thrive in a variety of bodily fluids. It is found in its highest concentrations in blood, vomit and feces. But coming into direct contact with semen, vaginal fluids, saliva or even sweat could still be risky while a patient is symptomatic. (Although it’s not likely patients in the throes of illness would be engaging in sex. And live Ebola virus, according to WHO, has never been isolated in human sweat.) Just how infectious those fluids may be after recovery, however, remains a series of question marks. Studies in this area have been extremely small and continue to be largely inconclusive. Thus far, there are no recorded cases of sexual transmission of Ebola. With more than 13,500 cases currently in west Africa right now, however, public health officials do not want to take any chances.



Bruce Ribner, the clinician who led the Emory University Hospital team that treated patients Kent Brantly and Nancy Writebol, said in a recent interview with Scientific American that although studies have shown Ebola patients shed genetic material from the pathogen into their sexual fluids there is scant evidence they are often shedding viable virus that could infect others. Yet even Ribner advised his patients about the recommended CDC guidelines of not having unprotected sex for three months. For now, it’s better safe than sorry.