Basic facts


Infections with Ebola viruses originating from Africa cause a severe disease in humans called Ebola virus disease. There are five species of the genus Ebolavirus (Filoviridae family): Zaïre ebolavirus, Sudan ebolavirus, Reston ebolavirus, Taï Forest ebolavirus and Bundibugyo ebolavirus [8,9]. The current outbreak in West Africa is caused by Zaïre ebolavirus. A concurrent EVD outbreak was declared on 26 August 2014 in the Democratic Republic of Congo. The two outbreaks are not connected [10].

Ebola viruses are biosafety level-4 pathogens (BSL-4; risk group 4) and require special containment measures and barrier protection, particularly for healthcare workers. The viruses can survive in liquid or dried material for many days [11] .They are inactivated by gamma irradiation, heating for 60 minutes at 60 °C or boiling for five minutes, and are sensitive to sodium hypochlorite (bleach) and other disinfectants [12,13]. Freezing or refrigeration will not inactivate Ebola viruses [14,15].

The incubation period (the period between infection and first symptoms) is usually four to ten days but can be as short as two days and as long as 21 days. The case–fatality ratio for Zaïre ebolavirus infections is estimated to be between 44% and 90% [16]. Ebola viruses are highly transmissible by direct contact with infected blood, secretions, tissues, organs and other bodily fluids from dead or living infected persons [17] . Transmission via inanimate objects contaminated with infected bodily fluids (fomites) is possible [18]. The principal mode of transmission in human outbreaks is person-to-person transmission through direct contact with a symptomatic or dead EVD case (Table 1). Airborne transmission has not been documented [19].

The probability of transmission is considered low in the early phase of human disease (i.e. the prodromal phase) [16]. Risk of transmission may increase with transition to later stages of the disease. During an Ebola outbreak in DRC, the most important risk factor was direct physical contact with an infected sick person. The risk was higher with exposure to bodily fluids during the late stages of the disease [20]. EBOV is shed in a wide variety of bodily fluids during the acute period of illness [21,22]. A marked difference in viral load, especially early in the course of disease, was previously observed with Zaire ebolavirus between survivors and non-survivors with high levels of viraemia associated with poor outcomes [21].

Burial ceremonies and handling of dead bodies play an important role in transmission [23]. Ebola virus genome has been detected in semen up to 91 days after onset of disease [24] , and replicative Ebola virus has been detected in semen 41 days after onset of disease [17,25].

Levels of risk of transmission of Ebola viruses according to type of contact with an infected patient:

Type of contact

Type of contact

Low risk

Casual contact with a feverish but ambulant and self-caring patient, e.g. sharing a seating area or public transportation; receptionist tasks.

High risk

  • Close face-to-face contact (e.g. within one metre) without appropriate personal protective equipment (including eye protection) with a probable or confirmed case who is coughing, vomiting, bleeding, or who has diarrhoea; or has had unprotected sexual contact with a case up to three months after recovery.
  • Direct contact with any material soiled by bodily fluids from a probable or confirmed case;
  • Percutaneous injury (e.g. with needle) or mucosal exposure to bodily fluids, tissues or laboratory specimens of a probable or confirmed case.
  • Participation in funeral rites with direct exposure to human remains in or from an affected area without appropriate personal protective equipment.
  • Direct contact with bushmeat or bats, rodents, primates, living or dead in/from affected areas.