Mariam’s doctors, it turned out, had reason to be concerned: A blood test revealed the typhoid strain Mariam carried was resistant to five classes of antibiotics typically used to treat the infection. Although resistance to three classes of typhoid medication—formally known as “multidrug-resistant” typhoid—had become common in Pakistan in recent decades, this extreme level of resistance was much worse. Mariam’s infection was designated “extensively drug resistant,” or XDR—meaning it would only respond to one powerful, broad-spectrum class of antibiotics: azithromycin, which is considered the last line of defense against typhoid. Mariam, who fell ill in November 2016, had the dubious distinction of becoming the second confirmed case of XDR typhoid in an outbreak that would soon roil much of southern Pakistan. Around the time Mariam was admitted to a hospital, another child was brought to the same ward and testing revealed the same affliction. The next week there were two more cases, and by the end of 2016 Hyderabad’s doctors had seen more than a dozen people with the extensively drug-resistant strain. Since then there have been more than 800 lab-confirmed cases, according to tracking by Aga Khan University (A.K.U.) in Karachi. And that number is widely considered an underestimate; many cases likely go undiagnosed or uncounted.

Infectious disease experts say Pakistan is in the grip of the world’s first outbreak of XDR typhoid, and they suspect the country’s abysmal sewage and water systems are the root cause for its spread.

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