Scientists have found one disease that doesn't easily spread through secondary infections: MERS.
People with MERS, or Middle East Respiratory Syndrome, rarely infect household members, and the cases of secondary infection from MERS are usually mild cases.
In a study, published in The New England Journal of Medicine, notes that MERS can be spread, but the disease is generally mild and occurs in less than half of patients.
"If less than half of infected patients transmit the virus to contacts, such as in this study, we can be pretty sure that this virus will not be able to start an epidemic in humans," said co-author Christian Drosten.
According to the CDC, the illness was first reported in 2012 in Saudi Arabia. It is caused by a coronavirus called MERS-CoV.
MERS has infected more than 850 people since 2012 and has killed 333 people worldwide. The virus can be lethal, Drosten said, and it kills up to 30 percent of first-generation cases of the disease.
In the study, researchers tested 280 family members and contacts of 26 MERS patients. They found only 12 probable cases - a 4 percent secondary transmission rate.
Out of those who showed indications of MERS, many were mildly sick and some didn't have symptoms at all. Researchers caution, however, that the transmission rate may be higher in closed settings such as hospitals or close family homes.
Silent, asymptomatic patients are particularly dangerous because they can transmit the virus to other people.
"These viruses can cause serious human-to-human transmission chains, but they don't in normal situations such as household contacts as investigated here," Drosten said.
There is currently no treatment for MERS nor are there vaccines that prevent MERS. The MERS virus requires that people be in close contact to pass the infection.
Researchers say, however, that the number of secondary infections could be more than 12 out of the 280, because some family members did not provide blood samples for a second test.
MERS is thought to have originated in camels. Symptoms of MERS include coughing, fever and pneumonia.
So far, MERS has only been found in countries in or around the Arabian Peninsula. There have been two cases in the US but they were not linked and they were imported from Saudi Arabia.
