E-cigarettes generate toxic chemicals similar to those found in tobacco and may harm the lungs and immune system, new research suggests.
The findings, from a study of mice, indicate that "vaping" is far from being a safe alternative to smoking tobacco.
In the experiments, mice exposed to e-cigarette fumes suffered mild damage to their lungs and became far more susceptible to respiratory infections.
Their immune responses to both viruses and bacteria were weakened by such an extent that some animals died.
Scientists also found that e-cigarette vapour contained "free radical" toxins similar to those found in cigarette smoke and air pollution.
Free radicals are highly reactive molecules that can damage DNA and cell membranes.
Professor Shyam Biswai, from Johns Hopkins University in the US, who led the research published in the online journal Public Library of Science ONE, said: "Our findings suggest that e-cigarettes are not neutral in terms of the effects on the lungs.
"We have observed that they increase the susceptibility to respiratory infections in the mouse models. This warrants further study in susceptible individuals, such as COPD (chronic obstructive pulmonary disorder) patients who have switched from cigarettes to e-cigarettes, or to new users of e-cigarettes who may have never used cigarettes."
For two weeks, mice were exposed to e-cigarette vapour in amounts equivalent to the doses inhaled by humans.
The animals were then infected with pneumonia bacteria or a strain of flu virus.
Compared with other non-exposed mice, they were much more likely to develop a weakened immune response.
Co-author Dr Thomas Sussan, also from Johns Hopkins, said: "E-cigarette vapour alone produced mild effects on the lungs, including inflammation and protein damage.
"However, when this exposure was followed by a bacterial or viral infection, the harmful effects of e-cigarette exposure became even more pronounced.
"The e-cigarette exposure inhibited the ability of mice to clear the bacteria from their lungs, and the viral infection led to increased weight loss and death, indicative of an impaired immune response."
The study is thought to be the first to investigate an animal's response to e-cigarette inhalation.
E-cigarettes generated just 1% of the amount of free radicals in tobacco smoke, but this still posed a potential health risk, said the researchers.
"We were surprised by how high that number was, considering that e-cigarettes do not produce combustion products," said Dr Sussan.
"Granted, it's 100 times lower than cigarette smoke, but it's still a high number of free radicals that can potentially damage cells."
E-cigarette sales in the US are expected to overtake sales of cigarettes in the next ten years.
In 2013, more than 250,000 American teenagers who had never smoked a cigarette reported using e-cigarettes.