A demonstrator in Boston, vapes during a protest at the Massachusetts State House against the state's four-month ban of all vaping product sales, October 3, 2019.

The number of cases of a deadly vaping illness continues to rise with 37 confirmed deaths and more than 1,800 sickened throughout the U.S., the Centers for Disease Control and Prevention said Thursday.

The total number of probable cases is now at 1,888, with 284 new cases diagnosed and three new fatalities over the last week, according to CDC data compiled through Tuesday. Patients have been found in 49 states as well as D.C. and the Virgin Islands, according to the CDC.

Public health officials still don't know what's making people sick.

The CDC is tentatively calling the illness EVALI, short for e-cigarette, or vaping, product use associated lung injury. Most patients have reported vaping THC, the active ingredient in marijuana, according to the CDC. National and state findings suggest products containing THC, particularly those bought off the street or other informal sources like through illicit dealers, are linked to most of the cases, the CDC said.

Dr. Anne Schuchat, the CDC's principal deputy director, told reporters on a conference call last week that officials are not just looking at what's being vaped but also whether the heating process in e-cigarettes could be playing a role in the illnesses.

E-cigarettes are generally thought to be less harmful than traditional cigarettes. However, public health officials say the long-term health risks are unknown.

"There could be more than one cause," she said.

Schuchat also said the number of cases linked to vaping appears to be "leveling off or even declining," adding the illness is "serious and potentially fatal, but it is preventable."

Schuchat warned that public health officials still don't know how the flu or other respiratory illnesses circulating during the winter may impact patients with a vaping-related lung illness.

The CDC is recommending that consumers stop vaping, particularly THC and especially anything bought off the street.