Ebola Virus: How Contagious?
Pertussis (whooping cough)
Droplets, airborne -- reproductive rate 12 to 17
Whooping cough is highly contagious. The bacteria that cause the disease attach to the tiny hair-like extensions (known as cilia) in the respiratory system, leading to the violent coughing that can make it hard to breathe.
It's spread when infected people cough or sneeze and others breathe in the bacteria.
Vaccination protects people, but outbreaks still happen. In 2012, for instance, more than 48,000 people with whooping cough were reported to the CDC, with 20 deaths. Most deaths were in infants younger than 3 months. Vaccination should begin at age 2 months, the CDC advises. Adults, especially those who will be around infants, also need a pertussis booster.
Respiratory droplets -- reproductive rate 6 to 7
Diphtheria is an infection caused by bacteria that are spread from person to person, usually by coughing or sneezing.
Diphtheria is not typically a concern in the U.S., Adalja says, due to routine vaccination. When it does occur, diphtheria can kill 1 in 10 affected, according to the CDC.
Person to person -- reproductive rate 5 to 7
The polio virus spreads from person to person and invades the brain and spinal cord, sometimes leading to paralysis.
The virus is spread from the stool of an infected person or from droplets from a sneeze or cough. Toys and other objects contaminated with the virus can also spread the disease.
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.