Good morning.

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Today’s introduction comes from Thomas Fuller, the San Francisco bureau chief.

The first drenching rains of the season, an atmospheric river that came ashore this week, flooded roadways and snarled traffic in the San Francisco Bay Area on Thursday. For one group of residents, the region’s homeless, the rains were more than just an inconvenience.

Damp and cold conditions bring influenza and infections to the crowded tent encampments that have swelled in recent years as soaring property prices have sent more people onto the street.

“We have camps where we can have 50 to 100 people living in proximity,” said Dr. Jason Reinking, a medical doctor who focuses on the homeless as part of a program run by the Roots Community Health Center in Oakland. “Viruses or bacteria can spread quite aggressively.”