Adam Toren remembers the last time he crawled into bed, pulled the covers over his head and took off sick from work. “My whole family got wracked by the flu around 2006,” said Mr. Toren, a tech entrepreneur and writer. “Hopefully, that was the last time.”

Thirteen years later, Mr. Toren, who lives in Phoenix, said he has not missed another day of work from illness, a streak of which he is exceedingly proud. To keep it going, “I monitor my sleep cycles,” he said , laying out the things he believes keep him healthy. “I don’t drink coffee. I drink specialty teas — Gyukuro, a Japanese tea. I take turmeric and resveratrol,” a supplement.

Mr. Toren’s attendance streak may be an outlier, but more and more, the sick day is disappearing from the office vocabulary, even as we hit peak flu season. Once, a sick day was just that — a day away from work to focus on recovery (or at least pretend to: Think “Ferris Bueller’s Day Off”).

But in recent years, it has become something murkier in definition and more reflective of our highly competitive, 24-7 work lives. The shifting definition and expanding mobility of the office — thanks to remote work and the rise of contractors in the gig economy — is also making the sick day somewhat passé , at least for some jobs.