Some 200 people have come down with serious respiratory illnesses in recent months that are linked to street-purchased vape products. The Centers for Disease Control has now taken the unusual step of issuing a warning about vaping, despite there being no single product or ingredient that's been identified as the culprit.

"E-cigarettes can contain harmful or potentially harmful substances, including nicotine, heavy metals (e.g., lead), volatile organic compounds, and cancer-causing chemicals," the CDC writes, adding that some vape pens and cartridges "may be modified for uses that could increase their potential for harm to the user."

The warning specifically points to e-cigarettes and vape products with cannabis in them (THC or CBD), but it suggests that some illnesses have presented themselves after only tobacco use. It further says, "Youth, young adults, pregnant women, as well as adults who do not currently use tobacco products should not use e-cigarettes."

As the New York Times reports, there have been 215 cases of severe respiratory illnesses in 25 states since late June, "some with lingering lung problems that required use of a ventilator or extensive monitoring in intensive care units." The CDC also notes the case of one patient in Illinois who died after hospitalization from an e-cigarette-related illness.

In central California, seven people were hospitalized in Kings County with illnesses that a doctor there connected with bootleg or street-purchased cannabis pens or cartridges. The cannabis blog Leafly explained that "empty vape cartridges come in from China," which are then filled by black-market sellers in the US. "Producers then fill the carts with raw THC oil cut with agents such as propylene glycol, vegetable glycerin, medium-chain triglycerides (MCT) oil, or more exotic chemicals." Solvents must be used in order to make THC or nicotine inhalable

The CDC's warning suggests a range of possible causes for the illnesses, and it recommends to doctors and clinicians that they make every effort to determine the brand and source of the e-cigarette product any new patient may have used, and to get their hands on the product itself for further testing.

The FDA is reportedly testing 80 samples taken of vape products in these cases, but has not released any results.

"I think this is probably going to be associated with illegal products," says former FDA commissioner Dr. Scott Gottlieb, speaking to the New York Times. "It’s not like the major manufacturers have suddenly changed their ingredients. It’s probably something new that has been introduced into the market by an illegal manufacturer, either a new flavor or a new way to emulsify T.H.C. that is causing these injuries."

Meanwhile, local e-cigarette giant Juul is fighting a local fight in San Francisco with an expensive ad campaign promoting a ballot measure to overturn the city's ban on e-cigarette sales. On Wednesday, Senator Dianne Feinstein filed a proxy vote voting "no" on the SF Democratic Committee's proposal to endorse Juul's measure, Proposition C.

Juul Labs CEO Kevin Burns went on CBS This Morning this week in an apparent PR campaign, trying to get ahead of the "vaping is bad" story by simply telling everyone who isn't already a smoker not to vape. "Don’t vape. Don’t use Juul. Don’t start using nicotine if you don’t have a preexisting relationship with nicotine,” he said. "Don’t use the product. You’re not our target consumer."

Related: Juul CEO Full-On Apologizes For Teen Vaping As Vape Ban Ballot Fight Heats Up