Ebola Virus: How Contagious?
Most people infected don't have any symptoms. Others have flu-like symptoms that go away.
Only 1 in 100 people infected develop the weakness or paralysis, according to the CDC.
Of those people who are paralyzed, up to 10% die when the paralysis affects the breathing muscles. Vaccination has wiped out polio from some, but not all, of the world. Only three countries in the world -- Nigeria, Pakistan, and Afghanistan -- have not stopped the spread of polio, according to the WHO. Stumbling blocks have included resistance to vaccinations and the reluctance of some leaders to back vaccination efforts.
Droplets of saliva, mucus; contaminated objects -- reproductive rate 4 to 7
Mumps is a viral illness. It's spread by droplets of mucus or saliva when an infected person coughs, sneezes, or talks.
Vaccination prevents the disease, but outbreaks have still been seen, including in the U.S. Most people who get mumps recover fully, the CDC says.
Sharing needles, sexual contact -- reproductive rate 1 to 4
HIV is the virus that can lead to AIDS. The body can't get rid of the virus, so once infected, a person has HIV for life.
HIV is spread mainly by having sex with or sharing needles or other drug equipment with an infected person.
The estimated reproductive number, 1 to 4, can vary greatly, Adalja says. It would typically be much lower if someone infected with HIV is on an antiretroviral drug, does not inject drugs, and does not take part in other risky behaviors, he says.
AIDS is still a killer in the U.S. According to the CDC, 15,500 people with AIDS died in 2010, the latest year for the statistics.
Bodily fluids, exposure to contaminated needles and other objects -- reproductive rate 1 to 4
Ebola was first discovered in 1976. Outbreaks have surfaced from time to time ever since. Ebola is particularly deadly, though. In the current outbreak, about 60% of those infected have died, according to the CDC.
Experts believe the virus hosts are animals, probably bats.