Over the next few weeks officials at the World Health Organization (WHO) face a tough and politically charged call. The Muslim month of fasting, Ramadan, begins July 9 and could draw as many as two million people from around the globe to the holy sites of Saudi Arabia in a pilgrimage called umrah. But a new disease, called Middle Eastern respiratory syndrome, or MERS, could threaten them.

Infectious disease control at mass gatherings is always a challenge, but this year even more so. Saudi Arabia is currently waging battle with MERS, yet it has released only the barest of details that scientists or public health officials could use to try to prevent its spread within Saudi Arabia or around the globe. In early May Saudi officials startled the world by announcing 13 new cases over the course of a few days. Since the start of May there have been 38 new cases worldwide—31 of them in Saudi Arabia—and 20 of the victims have died. With virtually no clues to draw on about where the virus lives in nature and how people contract it, WHO is trying to figure out what guidance to give those pilgrims, and the countries they will return to, about how to avoid infection and the international dissemination of a devastating new illness.

MERS triggers severe pneumonia and kidney failure in some cases. It is a cousin of SARS, severe acute respiratory syndrome, which broke out in mainland China in late 2002, spread from there to Hong Kong in 2003, and was then transported in the lungs of international travelers to Singapore, Hanoi, Toronto and other cities. Health officials do not want to pull out the big hammers used during the SARS outbreak, such as WHO travel advisories that urged the world’s citizens to avoid infected hubs such as Hong Kong and Toronto. On the other hand, no one wants umrah and the even larger hajj pilgrimage that will follow in October to trigger a pandemic.

The new virus was first isolated in June 2012. But its existence came to the world’s attention only weeks before last October’s hajj, when an Egyptian infectious diseases specialist who had been working in Saudi Arabia’s second largest city, Jeddah, reported that he had treated a man who died from an infection caused by a new coronavirus. Whether MERS has or can gain the capacity for sustained person-to-person spread is unknown. Kamran Khan, an infectious diseases physician who researches global flight patterns as a means of predicting disease spread, has had a worried eye on the Muslim religious calendar for some time. “We still don't have a good idea where this (virus) is coming from, so taking measures to mitigate risks are constrained,” says Khan, who works at the Saint Michael’s Hospital Keenan Research Center in Toronto.

Coronaviruses such as MERS, SARS and numerous others are named for the hallmark halo, or crown, they appear to sport in their outer shells. Many infect bats; the few that infect people cause illnesses ranging from the common cold to the severe lung devastation seen with many MERS cases, forcing patients to undergo mechanical ventilation. MERS has not yet evolved to spread as well as SARS can. And SARS, which was no wimp, killed about 11 percent of cases before it disappeared in 2004.

Last fall and in the early part of 2013 MERS infections popped up sporadically in a variety of places. Testing of samples from an April 2012 outbreak in Jordan revealed the virus had killed two nurses there. Three men in a family in the Saudi capital, Riyadh, appeared to have passed the virus to one another. Sick people from Qatar and the United Arab Emirates were medivacked to the U.K. and Germany. And more recently tourists have taken the infection to the U.K., France, Tunisia and Italy.

The affected Arabian Peninsula countries have not been particularly forthcoming with information, and global health experts have yet to hit on the right strategy for persuading officials to get serious about finding the source of the infections or the scope of the illness in people. An outbreak of H7N9 bird flu virus in China at the beginning of April also distracted attention from MERS.

The latter virus, however, would not be ignored for long. The 13 new infections in early May were linked, arising in dialysis patients treated in Al Moosa Hospital at the Al-Ahsa oasis in the kingdom’s Eastern Province. Infection in hospitals is how SARS took off, so word that an institution—or as sources suggest, several institutions—were epicenters of the outbreak raises the level of concern.

Donald Low, a microbiologist at Mount Sinai Hospital in Toronto who became a SARS expert in 2003, expressed hope that the Al-Ahsa outbreak would “put their feet to the fire to get serious about this.” Low has been worried about the possibility that superspreaders will emerge, as they did during SARS. Most people who contracted SARS passed the virus to at most one other person. But some SARS patients infected large numbers of people. One patient in Singapore infected 62 others; a woman who fell ill in the early days of the Toronto outbreak infected 44. With SARS, superspreaders turned a virus that likely would have burned itself out into a global outbreak that claimed 916 lives.

Has there been a superspreader in Saudi Arabia? If so, Saudi authorities have not revealed it. But it is evident that infections are being detected at a more rapid pace. At WHO’s annual meeting—the World Health Assembly—in late May, the Saudi delegation was given what amounts to a diplomatic dressing down, with Director General Margaret Chan lauding China for its handling of the H7N9 outbreak and demanding that countries with MERS cases act as good global citizens and share information in a timely, complete manner. The next day Saudi Arabia announced five more cases in a three-line statement, which revealed only that victims ranged in age from 73 and 85; all had chronic diseases and lived in the Eastern Province.

Infectious disease experts are aghast that this late into MERS’s spread the world still has no idea what puts people at risk of infection, how long the incubation period is, when people are contagious or whether there are mild cases that are being missed because surveillance is focused on finding sick people in hospitals. They put the problem squarely at the feet of the Kingdom of Saudi Arabia (KSA), which accounts for 41 of the 55 infections to date. Says Michael Osterholm, director of the Center for Infectious Diseases Research and Policy at the University of Minnesota: “The European countries have largely done an exemplary job of investigating and following up on the cases [that have been exported there]. Now, either the Middle Eastern countries, particularly KSA, have not, or they’re just withholding information, for whatever reason. And in a situation where this represents a potential global pandemic, that is inexcusable.”

Scientists also have no sense of whether the virus has changed over time. Genetic sequences of only four viral isolates have been placed in GenBank, the open-access sequence database run by the National Institutes of Health’s National Center for Biotechnology Information. The most recent of the genetic blueprints dates to an infection that occurred in February. No sequences from the flurry of recent cases have been released. In fact, except for the sequence of the first spotted case—the man from Jeddah—no Saudi sequences have been placed in the public domain. The kingdom’s deputy minister of health, Ziad Memish, has promised that sequences will be shared.

This week an international team of experts convened by WHO has gathered in Saudi Arabia to make headway in prying information out of the country. With the clock counting down to Ramadan, they have little time in which to answer key questions about the disease—answers needed to help safeguard the umrah pilgrims, and the rest of the world.