Anne Schuchat, director of the Centers for Disease Control (CDC), testifies during a House Energy and Commerce hearing in Washington, D.C., on Wednesday, Sept. 25, 2019.

The Centers for Disease Control and Prevention is grappling with new cases of a deadly vaping lung disease every day, and the agency expects this week's total number of illnesses to increase "considerably," a top CDC official told Congress at a hearing Wednesday.

A mysterious vaping-related lung disease has sickened nearly 1,300 people and killed at least 26, the CDC reported last week. Those totals are expected to get "considerably" worse when the agency releases its updated case report later this week, CDC Principal Deputy Director Anne Schuchat told a House Appropriations subcommittee.

"This is extremely complicated and difficult. It's fatal or potentially fatal with half of the cases requiring intensive care," she said.

The approaching flu season complicates efforts to diagnose vaping cases, Schuchat said. The disease looks and sounds like pneumonia, which could confuse doctors. It also makes patients more vulnerable to other diseases, she said. The CDC last week issued new guidance to doctors encouraging them to assess patients for both the vaping-related disease and typical respiratory illnesses like influenza.

The CDC is tentatively calling the vaping-related illness EVALI, short for "e-cigarette, or vaping, product use associated lung injury." Patients who contract the disease could become more vulnerable to influenza and other respiratory illnesses common in the winter months.

"It's going to be a very challenging winter," Shuchat said.