South Korean authorities are tracking the health of more than 150 citizens and foreigners who entered the country from regions that have experienced outbreaks of the Ebola virus.
 
The Ministry of Health and Welfare and the Centers for Disease Control and Prevention said Thursday that they have either tracked or are monitoring the health of 168 South Koreans and foreigners.
 
The 168 people have departed or passed through the four South African countries hit by Ebola, which include Guinea, Liberia, Sierra Leone and Nigeria.
 
Health authorities began monitoring the travelers in April after a case of Ebola broke out in Guinea in March.
 
Out of the 168 people, 21 were confirmed to have no health problems, while the remaining 147 are still being monitored.
 
Such efforts come as the incubation period of the Ebola virus ranges from two to 21 days.
 
The number of people being monitored is about eight times larger than the figure the disease control center unveiled last Friday.