Ebola returns to Liberia — three new cases reported this week in the previously Ebola-free country. People are questioning whether Liberia was really free of the disease to begin with. Liberian health officials reported the death of a 17-year-old boy from Ebola on Sunday, and two other cases of Ebola in people who were with the boy when he died. Liberia is now monitoring more than 100 people who had contact with the boy who died, and this number is expected to increase.

New cases are the first in Liberia since the country was declared free of the disease on May 9. Health officials typically wait 42 days after the last case of infection has resolved to declare a country Ebola-free, because this is twice as long as the 21-day incubation period (the time it takes for a person infected with the virus to show symptoms) of the virus.

Even after this declaration, Liberia remained at risk for Ebola because two bordering countries — Guinea and Sierra Leone — were still reporting new cases.

Liberian health officials are not sure where these new cases came from. The teenage boy reportedly did not travel out of the country before he got sick, and he had not come into contact with people from Guinea or Sierra Leone, according to WHO.

New cases raise questions about whether Liberia was actually Ebola free during May and June, or if there was a "shadow epidemic" of cases that went undetected by officials. It's possible that the boy had contact with an infected animal, such as a nonhuman primate or a bat.

Another source could be sexual transmission — the virus has been shown to remain in the semen of survivors for months.There are also reports that some of the new Ebola patients in Liberia had previously eaten a dog. However, it's unlikely that these patients became infected with Ebola because they ate dog meat, as the virus has never been shown to spread this way.

The new cases show that "it's important, even if a country like Liberia is declared free of Ebola," that the disease still be considered as a possible illness in people who are sick.

 In the new case, the teenage boy was treated for malaria, and was only diagnosed with Ebola after his death.