The United States has confirmed a third case of Middle East respiratory syndrome (MERS), the deadly illness that has killed more than 150 people globally.

But despite the relatively high fatality rate of the disease, increased detections may moreover reflect the strenuous effort of the medical community to track the virus as it spreads.

Health officials are still learning about MERS, caused by a coronavirus first reported in Saudi Arabia in 2012 that has gone on to kill almost a third of the hundreds of known people to have come down with symptoms.

The source of the disease remains unknown, though doctors believe it likely transferred to humans from a camel or another animal in Saudi Arabia, according to the Centers for Disease Control and Prevention (CDC). Cross-species infection, or zoonosis, has led to other health outbreaks. SARS, which killed hundreds in the early 2000, for example, is believed to have started with bats.

Most people who have been confirmed to have the MERS infection develop severe acute respiratory illness and show flu-like symptoms such as fever, cough and shortness of breath, according to the CDC. The contagious virus spreads airborne, making it easy to spread from person to person, putting health care workers at risk in particular.

In Saudi Arabia, one-third of the 500-plus confirmed cases of MERS were reported in health care workers, according to World Health Organization (WHO).

“It’s very difficult to do all of the health care worker protective things all of the time that you’re doing your work caring for patients,” Amy Behrman, internist and director of Occupational Medicine Services at the Hospital of the University of Pennsylvania, told Al Jazeera. “It takes tons of personal vigilance towards one’s health and towards one’s coworkers to keep everyone protected.”

Health care officials recommend that anybody who is suspected of having MERS be put on “total precaution.”

“That means a combination of everything we need to do to prevent illnesses from spreading in a health care setting: by contact, by blood, and most importantly and most likely in this case airborne, and that’s the hardest one to do,” Behrman said.

Such precautionary measures played an effective role in marginalizing the SARS virus in 2003.

The most recent confirmed case of MERS in the United States, an Indiana man who had contact with an earlier MERS patient, did not show any signs of the flu-like symptoms. He was part of an investigation of individuals who had contact with the Illinois man who was infected by the virus. Such investigations help locate individuals with traces of the virus that would not otherwise be known.

The Indiana man’s immune system, found with antibodies to the virus but not the virus itself, defeated the disease.

“Someone who — on a theoretical basis — has laboratory evidence of having been expose to the virus, but was not sick or contagious, is probably not something to be terribly concerned about except as a marker of the progressive spread of the disease into a population,” Behrman said.

His defeat of the disease implies that many more may have been infected with the MERS virus without showing symptoms, and their immune systems created antibodies to defend against it.

“It’s possible that as the investigation continues others may also test positive for MERS-CoV infection but not get sick,” David Swerdlow, who is leading CDC’s MERS-Coronavirus response, said. “Along with state and local health experts, CDC will investigate those initial cases and if new information is learned that requires us to change our prevention recommendations, we can do so.”

Since 2012, more than 160 people have died from MERS in Saudi Arabia alone. Two annual Muslim pilgrimages to Mecca have taken place since 2012 without significant spread of the virus to the millions of worshippers. However, since the beginning of April, Mecca has seen an increase of confirmed cases with 21 — including one Turkish pilgrim — and four deaths, the Associated Press reported. The next pilgrimage is in October.