Part 2:

When the Black Death exploded in Arabia in the 14th century, killing an estimated third of the population, it spread across the Islamic world via infected religious pilgrims. Today, the Middle East is threatened with a new plague, one eponymously if not ominously named the Middle East respiratory syndrome (MERS-CoV, or MERS for short). This novel coronavirus was discovered in Jordan in March 2012, and as of June 26, there have been 77 laboratory-confirmed infections, 62 of which have been in Saudi Arabia; 34 of these Saudi patients have died.

Although the numbers -- so far -- are small, the disease is raising anxiety throughout the region. But officials in Saudi Arabia are particularly concerned.

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.