Health authorities are trying to learn what is behind the vaping-related deaths and illnesses.

The number of people in the US to have died from illnesses linked to e-cigarette use has risen to at least 19, health authorities say, while more than 1,000 others have suffered lung injuries probably linked to vaping.

Key points:

• Health authorities are trying to determine if the outbreak is only happening now, or if earlier cases were misdiagnosed
• Only one case of lung injury has been reported outside the USA, making the outbreak more mysterious
• The US Centre for Disease Control said it looked like there might be a lot of "nasty things" in vaping products

Officials have yet to identify the cause for the outbreak, which dates back to March, and they are pursuing multiple lines of investigation.

A report by clinicians in North Carolina last month pointed to the inhalation of fatty substances from aerosolised oils, but a new study by the Mayo Clinic published this week found patients' lungs had been exposed to noxious fumes.

The Centres for Disease Control and Prevention (CDC) said 18 deaths in 15 states had now been positively linked to vaping, from a total of 1,080 cases of people sickened — a jump of 275 since last week.

And officials in Connecticut announced the first death in the state, bringing the total to at least 19.

The CDC attributed the sharp increase to a combination of new patients becoming ill in the past two weeks and recent reporting of previously identified patients.

"I think we really have the feeling right now that there may be a lot of different nasty things in e-cigarette or vaping products, and they may cause different harms in the lung," Anne Schuchat, a senior official with CDC, said.

Among a group of 578 patients interviewed on substances they had used, 78 per cent reported using tetrahydrocannabinol (THC), the primary psychoactive substance of marijuana, with or without nicotine products.

Follow this story to get email or text alerts from ABC News when there is a future article following this storyline.

Another 37 per cent reported exclusive use of THC products, and 17 per cent said they had only used products containing nicotine.

About 70 per cent of patients are male, and 80 per cent are under 35 years old.

E-cigarette use skyrocketing

E-cigarettes have been available in the US since 2006.

It is not clear whether the outbreak is only happening now — or if there were cases earlier that were wrongly diagnosed.

Initially conceived as a smoking cessation device, e-cigarettes have rapidly become popular with teenagers, with preliminary official data for 2019 showing more than a quarter of high school students using e-cigarettes in the past 30 days.

They were until recently perceived as a less harmful alternative to smoking because they do not contain the 7,000 chemicals in cigarettes, dozens of which are known to cause cancer.

Only one case of lung injury has been reported abroad, making the outbreak more mysterious still.

Canadian authorities said in September a youth had been hospitalised, but so far no other countries have reported anything similar.

Public and political opinion appears to be hardening, however, with US Government announcing in September it would ban flavoured e-cigarette products — which are particularly attractive to young people — in the coming months.

India has issued an outright ban on all e-cigarette products, as has the US state of Massachusetts.

AFP