As the global Muslim population continues to expand, conditions during the Hajj, the holy pilgrimage to Mecca that every Muslim must make at least once in their life are growing more crowded raising the likelihood that a serious infectious disease could easily spread. This tendency was on full display last week as serious health scares at airports in New York and Philadelphia were traced back to pilgrims who were returning from the Hajj.

Passengers who were traveling from Dubai to New York City (a group that amusingly included rapper Vanilla Ice) were quarantined for hours at JFK airport on Wednesday amid reports that more than 100 people aboard the plane reported feeling ill. In the end, 11 people were sent to an area hospital for testing. Of those, two tested positive for a particularly infectious type of influenza A virus, and one of the two was gravely ill with pneumonia. A third person tested positive for a cold virus, according to Channel News Asia.

On Thursday, two flights arriving in Philadelphia from Europe were screened by medical teams after 12 passengers reported flu-like symptoms. One of them had visited Mecca for the Hajj.

The CDC was not alerted in advance about the two flights that landed in Philadelphia from Paris and Munich, but several travelers had complained of illness, triggering a "medical review" of 250 passengers from those flights, a spokesman said. Twelve passengers were found to have sore throats and coughs, and one also tested positive for the flu, a CDC spokesman confirmed.

Roughly 2 million Muslims traveled to the Holy City of Mecca last month to take part in the Hajj. While the pilgrimage was largely free of catastrophes like the deadly stampede that killed 800 pilgrims in 2015, the sheer number of travelers prompted the New York Times to calculate roughly how long it would take for every living Muslim to complete the Hajj once in their life. The answer, the Times found, is roughly 581 years.

Per NYT, the number of pilgrims voyaging to Mecca peaked in 2012 at more than 3 million. Since then, the Saudi government has tightened quotas for other countries that send pilgrims based on the size of their Muslim populations.

And while the vast majority of pilgrims have probably returned home by now after completing the five-day journey two weeks ago, health officials should already be preparing for next year's Hajj, which could present a serious public-health threat given how pilgrims spend long hours walking in packed crowds until they arrive at the Kaaba in what one reporter once described as an "overwhelming mass of humanity."