“When it comes to responding to Covid-19, the response by large dialysis organizations has been inadequate, leaving some of the most vulnerable exposed to Covid-19, which is especially serious in people with end-stage renal disease,” said Dave Regan, president of the Service Employees International Union-United Healthcare Workers West, in a statement.

Representative Katie Porter, Democrat of California, also wrote both companies in late March to ask what steps they were taking. “We are concerned that DaVita, specifically, has failed thus far to understand the severity of this outbreak and the high transmission of the virus,” she said.

In particular, she criticized DaVita for going ahead with an annual meeting of medical directors in early March, where one of the attendees later tested positive for the virus. In its response to Ms. Porter, DaVita defended the decision as being appropriate at the time and said no one appears to have been sickened as a result.

Infection has traditionally been one of the industry’s biggest problems.

“The second leading cause of death among hemodialysis patients is infections,” said Dr. Alan Kliger, a Yale nephrologist who is leading efforts to control the spread of coronavirus among dialysis patients. As many as one in 10 patients die from complications from an infection, often at the site where a patient has a catheter.

According to an analysis of Medicare data by the research department of the union, six out of 10 facilities undergoing routine inspections last year were cited for deficiencies involving infection control. Examples would include a staff member failing to wear gloves during a procedure or to properly clean equipment.

At this juncture, with the highly infectious coronavirus, others are extremely concerned about the spread of the virus through inadequate supplies of special masks or gowns. “We don’t have enough of many of those protective equipments,” Dr. Kliger said.