Ebola Virus: How Contagious?
Airborne -- reproductive rate 12 to 18
The measles virus spreads through the air when infected people breathe, cough, or sneeze. Any person exposed to the virus who is not immune generally gets the disease. It is less common in countries with good vaccination coverage.
The virus typically grows in the cells lining the lungs and the back of the throat, leading to fever, runny nose, cough, and an extensive rash.
Measles was declared eliminated in the U.S. nearly 15 years ago, according to the CDC. Even so, outbreaks still happen. In 2014, 288 people were confirmed to have had measles as of May 23. No deaths have occurred this year in the U.S. from measles.
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.