Given that we really don't know where Ebola lives most of the time, it's hard to get a good sense of how the very first person in an outbreak (index case) gets infected. For past outbreaks, the index cases are largely unknown, but a few have been identified as having been in places where bats have been present, or the victims have been known to have interacted with dead animals.
Beyond the unknowns of how the first person got infected, we do know how the virus is transmitted otherwise. Human-to-human transmission occurs as a result of close contact with blood, secretions or tissues from patients where the virus can travel to a new victim through mucosal surfaces or breaks in the skin. So the African burial rituals we mentioned earlier where family members clean out the waste from dead bodies? Yeah, that's a bad idea when your loved one has died from Ebola. We also are aware of cases of human-to-animal contact resulting in infection. As far as we've seen, those known animals are gorillas, chimps and a type of antelope called a duiker.
Thankfully, we haven't learned of any cases where the virus passes to humans via aerosol contact — or through the air. In the Ebola outbreak on monkeys in the U.S., the Reston Ebola virus was able to pass through the air to infect many of the monkeys in the research facility (see sidebar). A 2012 study conducted on pigs and monkeys with the Zaire strain also showed transmission of the disease without direct contact, but as far as we know, this airborne transmission method is not linked at all to humans [source: Weingarti].
One of the easiest transmission methods to control is the passing of the virus through dirty needles. Reuse of needles at health care facilities has massively impacted the numbers of infected people in past outbreaks, but with knowledge of this as a transmission route for the virus, health care workers can take extra precautions to keep used needles from being used again.
Depending on how the virus gets transmitted, the outcome of how Ebola affects a patient can change. For example, incubation periods for the Zaire type of Ebola is three to six days for a patient who has been stuck with a needle versus five to nine days for a patient who has had contact exposure. And in the first known outbreak in 1976, 100 percent of the patients who had been infected via needle-stick succumbed to the disease, whereas about 80 percent of the cases due to contact were fatal [source: Feldmann].