KARACHI, Pakistan — First came the floods, as weeks of monsoon rains deluged neighborhoods across Karachi, sending sewage and trash through Pakistan’s largest city. Then came the long power outages, in some cases for 60 hours and counting.

And then it got worse: Karachi is now plagued by swarms of flies. The bugs seem to be everywhere in every neighborhood, bazaar and shop, sparing no one. They’re a bullying force on sidewalks, flying in and out of stores and cars and homes, and settling onto every available surface, from vegetables to people.

Flies and flooding can often go together, and Karachi is no stranger to either. But Dr. Seemin Jamali, the executive director for the Jinnah Postgraduate Medical Center, one of Karachi’s largest public hospitals, said this was the worst infestation of flies she had ever witnessed.