But those benefits only apply to employees of businesses with fewer than 500 employees, or the government, who are infected by the virus, quarantined, have a sick family member or are affected by school closings. Large employers are excluded, and the Labor Department will have the option of exempting workers at any company with fewer than 50 employees, if it determines that providing paid leave “would jeopardize the viability of the business as a going concern.”

Those exemptions could potentially exclude nearly 20 million workers. About 59 million Americans work for companies with 500 or more employees, according to the Bureau of Labor Statistics, and about 6.5 million of them do not have any paid sick days. Another 12 million work for companies with fewer than 50 employees and do not have paid sick days. (Some large companies have announced since the outbreak began that they will provide paid leave for workers who get the virus. Walmart said it would give up to two weeks of pay; Target said it would give 14 days.)

The bill also limits benefits to workers who are sick, subject to a quarantine or caring for a family member; it stops short of what some public health experts have called for — effectively paying any worker to stay home, in order to reduce the spread of the virus.

It also excludes “certain health care providers and emergency responders” from the benefit.

Companies covered by the policy will not foot the bill for paid leave. The government is providing new tax credits to both help offset the costs of accommodating the new mandate and the economic impact of the outbreak.

The paid leave provisions were among the most contentious elements of the legislation. Republicans objected to a proposal by Democrats to establish a permanent paid sick leave entitlement for all families, according to a Democratic aide familiar with the negotiations, who spoke on condition of anonymity to describe the private negotiations.