JOHANNESBURG — Ebola, the virus that killed more than 4,800 people in Liberia since the outbreak began in December 2013, is back after the West African country was twice declared free of the disease. And no one is sure why.

After a 15-year-old boy recently contracted the virus, his father and brother also tested positive for the disease. Liberian health officials have placed 153 people under surveillance, including 22 nurses and six patients from a hospital in Monrovia, the capital, according to Liberian health authorities.

The new Ebola cases in Liberia raise fears that neighboring Sierra Leone, recently declared Ebola free, may suffer similar setbacks in eradicating the disease. Guinea, the third West African nation where the recent outbreak was centered, has not reported a new case since the end of last month and has begun a 42-day countdown to be declared Ebola free. By last week, 28,634 people had contracted Ebola, and 11,314 had died from confirmed or suspected cases, according to the World Health Organization.

How the latest Ebola case was transmitted to Nathan Groote, the 15-year-old from the Monrovia neighborhood of Paynesville, remains a mystery.

It’s known that the virus can stay in survivors’ semen, ocular fluid, breast milk and spinal fluid for months after their recovery, according to the Centers for Disease Control and Prevention. And many of the survivors suffer enduring problems, such as joint pain, eye problems, headaches and other chronic health issues. But little is known about why the symptoms persist, the center says.

“The fight against Ebola is not over yet, but we must not lose hope and must continue the practices we used to beat Ebola before. We can win this battle again with your participation and support of the communities,” the Liberian Health Ministry said in a statement.

Liberia was declared Ebola free in May, only to see a 17-year-old boy and a woman die from the disease in July. In September, the World Health Organization again declared the country to be Ebola free.

Liberian Health Minister Francis Kateh told journalists that investigations were continuing into how the disease was transmitted. CDC officials will travel to Liberia to help determine the cause, Kateh told The Associated Press.