There have been more than 1,600 cases of the mysterious illness that is associated with vaping since May. Thirty-six people have died. The New York Times has a dedicated tracker. What exactly is causing these illnesses and deaths is still unclear, but the Centers for Disease Control and Prevention, which has been trying to assess a plausible culprit for months, recently confirmed what observers have long suspected: It seems to be tied to weed. “The data do continue to point towards THC-containing products as the source of individuals’ injury,” Anne Schuchat, the principal deputy director of the CDC said on Friday, according to the Washington Post.

For such a long time vaping seemed safe to much of the public. But doctors have long worried about unknown long-term harms, and as vaping got more popular with teens who had never smoked cigarettes, skepticism and backlash have been mounting. The sudden spate of illnesses, including the deaths, has not only been alarming, but it’s also served as fodder for critics who have wanted stronger regulations for years. But it’s still unclear what exactly about vaping is causing the illnesses. Sean Callahan, who specializes in complex lung diseases at the University of Utah, told me that the cause is “probably a very nuanced reaction.” Even the CDC’s latest announcement isn’t terribly clarifying. So, in the interest of better understanding what has been ruled out and what’s a prime suspect, I’ve rounded up a list of all the potential culprits and (unscientifically) estimated their likeliness.

Vaping in general: Despite all the warnings to stay away from vapes, this illness is not being caused by vaping at large. Yes, vaping is not good for you. Nicotine isn’t a neutral substance, plus vaping involves inhaling other chemicals with unknown risks. But as Lynn Kozlowski, a professor at the University at Buffalo who studies tobacco use, told me in September, “you have to keep in mind that nicotine has been vaped for years with no such adverse effects being reported.” Even that’s not totally accurate: As a report from Bloomberg News found, there seem to be at least 15 cases in which vaping caused acute lung damage before this epidemic. Still, that’s far from a typical outcome. And it’s important to remember that for smokers, vaping does represent a safer and healthier alternative for consuming nicotine.

Is it causing the vaping epidemic?: No.

Something like pneumonia: Infections like pneumonia can cause similar damage to what doctors are seeing with this illness. But such infections don’t occur in the middle of summer. Plus, it’s pretty easy to test for.

Is its causing the vaping epidemic?: No. But flu season will make weeding out these cases more of a chore.

The flavoring in vapes: Mango and gummy bear vapes are very easy to be concerned about: They appeal to teenagers, and to the already-hard task of figuring out the long-term effects of vaping, they add a whole additional mess of chemicals and reactions. “Because there are so many flavorings, it’s a difficult thing to nail down,” says Yasmin Thanavala, a cancer researcher at Roswell Park. But flavors have been around for as long as vaping has, so it would be surprising if they were suddenly now causing a problem. They’re an easy scapegoat, and President Donald Trump and others have used the current panic as a means to try to get rid of them entirely. But doing that is not likely to check the illness.

Is it causing the vaping epidemic?: No.

Contaminated Juuls: Earlier this year, Juul allegedly shipped 1 million contaminated mint pods, BuzzFeed News revealed this week. The claim comes from a lawsuit filed by a former high-ranking employee. The company told BuzzFeed that there had been a manufacturing issue but denied that this issue led to unsafe pods. The exact alleged contaminant is unspecified.

Is it causing the vaping epidemic?: It’s plausible that contaminated Juuls could be responsible for a few cases, but since they don’t contain THC, it’s unlikely that they are the main cause.

Vapes, but hotter: “I have talked to some patients who say that they modify their device to burn at higher temperatures,” says Chiarchiaro. A higher burn temperature means that chemicals like formaldehyde that form as an ordinary vape operates can be produced more quickly, possibly delivering a dangerous dose. It could also mean that harmful ingredients in black market vapes, or contaminants, are delivered more quickly.

Is it causing the vaping epidemic?: Perhaps! But it seems more likely that it’s a contributing factor, if anything, rather than the full cause.

Weed vapes with Vitamin E acetate: The majority of people with the vaping illness have reported using vape cartridges with THC, the stuff in pot that gets you high. So much so that Michael Siegel, a physician at Boston University who studies tobacco control, thinks that the CDC and medical professionals have spent a lot of time making this whole situation more muddy and complex than it needs to be. “I think that there is really a strong bias on the part of many of these help groups against vaping,” he says. To his eye, the clear culprit is the THC vapes, a position that the CDC has homed in on as well. (It’s true that not everyone who’s gotten ill has reported vaping THC, but it makes sense that young people would lie about their marijuana use.) Though the CDC notes that the only way to be totally risk-free is to avoid vaping products altogether, it makes a stronger recommendation to at least avoid vaping products that contain THC. But even focusing on THC leaves a mystery: What about THC vapes is suddenly causing lung injuries? A leading theory is Vitamin E acetate, a thickening agent that has “overtaken the THC black market,” according to Siegel. “It wasn’t something that was used prior to this—that would explain why this is happening now.” Inhaling the Vitamin E could be irritating to the lungs, causing inflammation.

Is this causing the vaping epidemic?: Maybe.

Weed cartridges that are otherwise contaminated: Perhaps other ingredients or contaminants in black market THC vapes are at least partially responsible. In tests commissioned by NBC News, 10 out of 10 bootleg THC vape cartridges contained harmful pesticides. (Though what dose of those pesticides is unclear; they weren’t present in samples of three legit cartridges.)

Is it causing the vaping epidemic?: Maybe.

Black market vapes: The reason most of the responsible products contain THC might be because THC vapes are more likely to be doctored than nicotine vapes, particularly in states where marijuana is illegal. It doesn’t account for cases in Oregon, where it is legal, though it could help account for the fact that some people claim they only vaped nicotine.

Is this causing the vaping epidemic?: Maybe to likely.

Not everyone agrees that THC products are definitively to blame, though. “I don’t feel very comfortable telling people, just stay away from THC,” says Callahan. Tom Jeanne, an epidemiologist at the Oregon Health Authority, notes that even in his state, where pot is legal and there’s less incentive to lie, some patients still say they’ve only used nicotine. That doesn’t ding the THC vapes’ top spot on this list, though. Jeanne notes that “we certainly agree that THC products seem to be the highest risk.” At least everyone can agree that the mystery is still unfurling.