People in Freetown, Sierra Leone, pass an Ebola-prevention banner, part of the country's campaign to eradicate the disease. (Aurelie Marrier d'Unienville/AP)

The Centers for Disease Control and Prevention warned late Tuesday that Ebola lingers in semen much longer than previously believed, underscoring how much we still don’t understand about the virus.

Researchers initially thought that once people survived Ebola, they were immune and could no longer get sick and transmit the virus to others, as is the case with many other infectious diseases we’re familiar with, such as chickenpox and even the bubonic plague. Then cases emerged such as that of Scottish nurse Pauline Cafferkey, who was thought to have recovered but then had several relapses. American physician Ian Crozier’s left eye turned from blue to green because of the virus that remained in his body.

Thousands of other Ebola survivors in West Africa are dealing with similar long-term symptoms.

One of the biggest mysteries about Ebola has been about viral persistence and how long fragments can remain in bodily fluids, such as breast milk, saliva, urine and semen.

The issue is one of the most critical in the aftermath of the Ebola epidemic that killed more than 11,000. Since shortly after March 2015, health officials have worried that sexual transmission could spark new outbreaks. That March was a quiet time in Liberia; the country had been declared Ebola-free. But then a woman in Monrovia became ill and died, and genetic sequencing showed that the virus she had was very similar to a virus carried by someone she had sexual intercourse with. That led to a mad scramble by scientists around the world to try to figure out what was going on.

An early study estimated that Ebola could stay in the semen for 70 days; another said 90 days. In October, scientists reported the stunning finding that it could remain for at least nine months.

The report out Tuesday in the Lancet Global Health journal finds that even that was a low estimate. The study, the largest to date to look at Ebola virus persistence in men who survived infection, comes from a Liberian public health program run in coordination with the World Health Organization and the CDC; it involves data collected between July 2015 and May 2016. It shows that of the 429 men who were tested, 9 percent, or 38 Ebola survivors, had fragments of the Ebola virus in their semen.

Of the 38, 63 percent had semen samples that tested positive for Ebola one year later. One man was found to still carry Ebola a shocking 565 days after he recovered from the illness. That’s a full year and 200 days.

While the test used can only detect the genetic material of the virus in the semen but not whether there is a live virus that can transmit the disease, the mere presence of the fragments has been enough to raise alarm bells for health officials.

Here's how the virus spreads and how contact tracing works to stop outbreaks. (Gillian Brockell/The Washington Post)

Moses J. Soka, coordinator for Ebola virus disease survivor clinical care at the Liberian Ministry of Health, said the work “demonstrates the importance of providing laboratory testing and behavioral counseling to empower survivors to make informed decisions to protect their intimate partners.”

An intriguing finding in the study that raises more questions than it answers is that men who were older than 40 were more likely to have a semen sample test positive. The researchers were not able to explain why this was the case.

Public health officials in Liberia, Sierra Leone and Guinea have put a lot of effort in recent months into providing education and counseling about safe sex to try to prevent any further spread of Ebola. In Liberia, Ebola survivors who are men and age 15 or older can get their semen tested each month.

Sexual transmission of another major public health threat — Zika — has also worried the scientific community in recent months. In February, the city of Dallas reported that a local resident was infected after having sex with a person who had traveled to Venezuela. Earlier this month, the CDC said a Maryland woman had contracted Zika virus from a male sex partner who showed no symptoms of infection. The takeaway from these cases, public officials have emphasized, is a message they have been promoting for years: Use condoms whenever possible.

This post has been updated.

Read more:

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