E-cigarettes contain toxic chemicals which could damage the lungs and immune system, scientists have discovered.
Experiments carried out on mice found e-cigarette fumes harmed the animals' lungs and made them more susceptible to respiratory infections.
Chemicals generated by the nicotine devices also weakened the immune system's response to viruses and bacteria, causing some mice to die.
Researchers found e-cigarette vapour contains "free radical" toxins similar to those found in air pollution and cigarette smoke.
Although the study said they generated just 1% of the amount of free radicals in tobacco smoke, these molecules can damage DNA and cell membranes.
The study was carried out by scientists at John Hopkins University in Baltimore.
Professor Shyam Biswai 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."
The study exposed mice to e-cigarette vapour equivalent to doses inhaled by humans for a period of two weeks.
Researchers then infected the mice with pneumonia bacteria or the flu virus.
The results showed the infected mice were far more likely to develop a weakened immune system.
Co-author of the report Dr Thomas Sussan 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."
Results of the study have been published in the online journal Public Library of Science ONE.