28 April 2014, West Africa, Guinea - Ebola disease control and prevention: contact tracing defined. This community health agent visits all of the persons in his villages who have been in contact with an Ebola disease patient. Every day for 21 days, the maximum incubation period for Ebola virus, each contact is checked for signs of illness. Those who appear to be ill are examined by specially trained nurses. Those who are not ill after 21 days are considered no longer at risk.