Meanwhile, in just a short time, millions of pilgrims will descend on Saudi Arabia to join in the Hajj. That’s even as the country is struggling to assess the severity of a new disease outbreak that could make SARS look like the common cold: MERS. With modern travel allowing people to sweep around the world in hours, pilgrims could unwittingly return home and spread this disease to the far corners of the planet and that’s got epidemiologists and public health specialists worried. A lot.

This fall, millions of devout Muslims will descend upon Mecca, Medina, and Saudi Arabia’s holy sites in one of the largest annual migrations in human history. In 2012, approximately 6 million pilgrims came through Saudi Arabia to perform the rituals associated with umrah, and this number is predicted to rise in 2013. Umrah literally means “to visit a populated place,” and it’s the very proximity that has health officials so worried. In Mecca alone, millions of pilgrims will fulfill the religious obligation of circling the Kaaba. And having a large group of people together in a single, fairly confined space threatens to turn the holiest site in Islam into a massive petri dish.

The disease is still mysterious. Little is understood about how it is transmitted and even less regarding its origins. But we do know that MERS is deadly, with a mortality rate of about 55 percent — a remarkably higher lethality than that posed by its close cousin, the severe acute respiratory syndrome (SARS) virus, which in 2003 terrified travelers across the globe but posed a fatality rate of only 9.6 percent. The MERS coronavirus is new to our species, so mild and asymptomatic infections seem to be rare, but the human immune response to infection is itself so extreme that it can prove deadly in some cases.

Like SARS, the MERS virus spreads between people via close contact, shared medical instruments, and coughing. Once inside the human lung, the MERS virus sparks a series of reactions that all but destroy normal lung function. Patients can descend into pneumonia so severe that they require machine-assisted breathing to stay alive, in as little as 12 days. Unlike SARS, the MERS virus is also capable of attacking the kidneys and can be passed on to others via exposure to contaminated urine. And for some of those who survive acute MERS, years of rehabilitation may be necessary, just like for some of the 2003 SARS victims.

And like back in 2003, when health officials worried about airplane travelers in confined spaces transmitting the virus across the globe, the hajj poses a unique risk of transmission, one that could catapult this still-small outbreak into a full-fledged pandemic. Containment will become nearly impossible as millions of pilgrims flock from virtually every country on the globe to the kingdom during the holy month. Indeed, MERS has already crossed continents; two suspected cases were reported in France as recently as June 12, and confirmed cases have been reported in Germany and Britain. The first patient in each of these cases had traveled in the Middle East before reaching his/her home destination, only then to be diagnosed with MERS.

MERS is a new and novel disease, but it’s hardly the only one experts are worried about. Saudi officials are also seriously concerned about the spread of polio, which while not natively present in the country, could be imported from those few remaining countries that have ongoing outbreaks. That could result in a spike of new cases in places that have eradicated the disease - and vaccination in these remaining places is tough because of a number of anti-vax rumors and conspiracies that run rampant.