By Kathleen Doheny
WebMD Health News
Reviewed by Michael W. Smith, MD
Aug. 6, 2014 -- Two Americans stricken with the Ebola virus amid a record outbreak in West Africa are now being treated in Atlanta, which has triggered fears of a potential outbreak in the United States.
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Infectious disease experts say they don't expect that to happen for several reasons. Ebola is hard to contract, they say, and good infection-control practices can stop its spread.
Many factors play into how contagious a disease is thought to be, say Jeff Duchin, MD, an infectious disease expert at the University of Washington, Seattle, and Amesh Adalja, MD, an infectious disease expert at the University of Pittsburgh.
Among those factors:
- How it's transmitted (airborne, bodily fluids, other)
- Infection-control practices in place
- Extent of contact an infected person has with others
- Percent of the population that has been vaccinated (if a vaccine exists)
To gauge how contagious different diseases are, experts take these and other things into account and estimate the average number of people likely to catch the illness from a single infected person. They call this the basic reproductive rate or number. The number is an average, a scientific guess, experts say, and it is likely to vary from country to country.
"I would anticipate the reproductive rate for Ebola in the U.S. to be zero," Adalja says.
By comparison, measles, diphtheria, and whooping cough are all airborne, and they can be transmitted by "just being in face-to-face contact with an infected patient, without touching them," Duchin says. When that person coughs or sneezes, others may become infected after breathing in the organisms.
Here are the estimated, overall, basic reproductive rates for Ebola and other infectious diseases, along with how they're spread.
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.
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 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 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.
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.
While Ebola and HIV have a similar reproductive number, they are different in many ways, Duchin says. "HIV and Ebola both are present in the blood, but the ways they infect cells, where they live in the body, are very different."
Compared to the airborne organisms spread by casual contact, "it takes effort to get infected with both of these viruses [HIV and Ebola]," Adalja says.
SOURCES: Jeff Duchin, MD, chair, public health committee, Infectious Diseases Society of America; professor of medicine, University of Washington; chief, communicable diseases program, Seattle King County Public Health Department. Amesh Adalja, MD, infectious disease specialist, University of Pittsburgh Medical Center; representative, Infectious Diseases Society of America. University of Michigan School of Public Health: "Basic Reproductive Rate." Centers for Disease Control and Prevention. World Health Organization.
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