NEW YORK (AP) — An outbreak of vaping illnesses. A surge in teens using electronic cigarettes. They're often spoken of in the same urgent breath, but it's not clear how — or even if — they are connected.

Following a shakeup at Juul Labs Inc., the largest U.S. seller of e-cigarettes, here are some questions and answers about vaping:

WHAT HAPPENED WITH JUUL?

The San Francisco-based company that controls about 70% of the market announced Wednesday that it will no longer run TV, print or digital advertisements for its e-cigarettes.

Juul, which rose to the top through viral marketing that promoted...