(CNN) -- Earlier this summer, Kelsey Mirkovic, a disease detective with the Centers for Disease Control and Prevention, entered a hut with her team in Gueckedou, Guinea, to speak with a man who had Ebola.
Their mission: to get the names of everyone he'd had contact with while he was ill, so that they could stop those people from spreading the disease.
"Who lives with you here? Who has eaten off the same plate as you? Who has bathed you and taken care of you?" they asked him.
Just his wife, the man answered.
Mirkovic and her team knew that wasn't true.
They knew he had children, and they knew that in West Africa, families and even neighbors eat off the same plate and bathe and care for sick people. They explained to the man how important it was to stop Ebola, and that his friends and family would be treated with respect.
Their pleas didn't work.
Mirkovic saw this scene play out over and over again. One of her colleagues at the CDC who's worked in Liberia says preliminary data shows they could be missing 40 to 60% of the contacts of known Ebola patients.
"This is one of the hardest parts of the response," said Dr. Brett Petersen, a medical officer with the CDC.
Mirkovic agrees. She says she understands why Ebola patients don't want to name names: There was a rumor going around the communities she worked in that getting on a contact list meant you would die -- and the deaths would happen in the same order as they appeared on the list.
"I understand they're scared," she said. "But it's very frustrating."
The Ebola outbreak in the West African nations of Guinea, Sierra Leone, Liberia and Nigeria has killed nearly 1,500 people. Just one contact left un-traced could go on to start a whole new line of Ebola transmission.
"It's like fighting a forest fire. If you leave behind even one burning ember, one case undetected, it could reignite the epidemic," Dr. Thomas Frieden, the director of the CDC, told reporters at a press conference earlier this summer.
"Contact tracing is a formidable challenge," said Dr. Margaret Chan, director-general of the World Health Organization. "In some areas, chains of transmission have moved underground. They are invisible. They are not being reported."
There's no solid number of how many contacts have gone missing. Petersen said the CDC arrived at the 40 to 60% number because in some communities, each sick person has only listed an average of two contacts -- and households commonly have five or six people.
Mirkovic, an officer with the CDC's epidemic intelligence service, said when she and her team felt patients weren't being honest, they would try to get information from neighbors or community leaders.
Sometimes that helped, and sometimes it didn't.
"Unfortunately, there's nothing we can do," she said. "We can't force them" to give contacts.
The World Health Organization estimates that 10% of contacts will go on to develop symptoms of Ebola. Occasionally, some of these Ebola cases go missing as well.
"Many families hide infected loved ones in their homes," according to a WHO press release issued Friday.
Mirkovic, who left Guinea in the end of July, said she felt that the situation might improve as health care workers gain more trust in the community.
But there's another problem with that: the availability of workers to follow up with contacts.
For example, in Sierra Leone, there are 2,000 contacts that need following, but the group Doctors without Borders says they've only been able to follow up with about 200 of them.
The group's teams in Sierra Leone and Liberia are "stretched to the breaking point" as the epidemic is "spiraling out of control," the group wrote in a press release.
CNN's John Bonifield contributed to this story.