NEW YORK — For Shannon Henderson, getting a cold or flu could be the difference between putting food on the table and going hungry.

As a part-time customer service representative at a Wal-Mart in Sacramento, California, Henderson is one of an estimated 40 million American workers for who calling in sick is a luxury. If they don’t work, they don’t get paid.

“I’m super afraid of getting sick,” said Henderson, 29, who slathers on hand sanitizer at work in hopes of fending off illness.

Paid sick leave is the next frontier in the fight for the country’s lowest earners. Some of the same workers’ rights groups that grabbed headlines recently by pushing companies for wage hikes are steering the conversation toward paid sick leave. The debate has caught the attention of governments and companies alike.

President Barack Obama is calling for federal legislation that would require companies to guarantee workers paid sick days. And since San Francisco started requiring that in 2007, nearly 20 cities and three states — Connecticut, Massachusetts and California — have passed similar measures. New York, Maryland and other states are considering laws too. And McDonald’s Corp. and Wal-Mart Stores Inc., which have announced wage hikes recently, are making changes to their paid sick leave policies.

“Paid sick days are a job issue,” said Ellen Bravo, executive director for Family Values (at) Work, a network of coalitions fighting to pass paid sick days and family leave policies. “When you don’t have sick pay, you get docked.”

The new focus comes amid wide disparities between the benefits received by the top and bottom rungs of the corporate ladder. Sixty-one percent of U.S. workers get at least one paid sick day, according to a national compensation survey of employee benefits conducted last year by the Bureau of Labor Statistics.