As the number of patients combating mysterious lung illnesses grows, state and federal public health officials are warning people to stay away from e-cigarettes and vape pens, especially those obtained off the streets.

As of Friday, public health officials were investigating more than 450 possible cases of severe pulmonary disease related to vaping, including five deaths, in 33 states, according to the Centers for Disease Control and Prevention. Investigators are looking into 60 possible cases in California, including one death.

Though the exact cause of the outbreak has not been determined, CDC officials said that the cases could be linked by some sort of chemical exposure, but that it is too early to single out one product or substance.

In the majority of the cases, patients reported recent use of THC, the psychoactive chemical in cannabis, officials said. A smaller group reported vaping only nicotine.

Public health experts recommend that people stop vaping while the investigation continues. If they develop symptoms, including cough, chest pain, shortness of breath and fevers, they should seek medical attention.

California Healthline reporter Ana Ibarra appeared Monday on “The Brian Lehrer Show” on WNYC to discuss what we know — and don’t know — about this growing number of vaping-related illnesses.