Ebola Virus: How Contagious?

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.

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.

What's more, Ebola is much less contagious than many other more common diseases. The virus, much like HIV or hepatitis, is spread through blood or bodily fluids and is not airborne.

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.

Measles

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.

Diphtheria

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.

Polio

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.

Mumps

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.

HIV

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.

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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.

Ebola Virus

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.

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