Lauren Caggiano had felt sick for days by the time she tested positive for the flu in an emergency room on a February afternoon. Hours later, she was in the intensive care unit. By 4 in the morning, she was on a ventilator.

Ms. Caggiano, a paralegal who lived in Oceanside, Calif., doted on her two dogs and had recently become a grandmother, died two days later. She was 49.

“You don’t really think, if you’re in decent health, that’s going to be what gets you,” her son, Brandon Salgado, said.

Many people recover from a bout of flu within a few days or a week. But every year, the virus still kills more than 36,000 people across the United States and sends hundreds of thousands to the hospital. The Centers for Disease Control and Prevention has said that this flu season has been especially severe.

Some of those who died were at greater risk for getting seriously ill because of underlying conditions or their age. Others, like Ms. Caggiano, were otherwise healthy before their infections. Some had not received the flu shot, which reduces but does not eliminate the risk of death. Some were hospitalized for weeks; others felt ill for only days before they died.

All of their deaths came as a shock to the people who knew them.

A Swift Decline

Part of what stunned Mr. Salgado was just how quickly his mother died.