Teenagers who had suffered bereavement or spent time in care as children were more likely to have depleted immune systems, scientists found, than others who did not experience that kind of trauma at a young age.

Prof Chris Coe, the University of Wisconsin-Madison, who led the study, said: "The immune system is not preset at birth. The cells are there, but how they will develop and how well they'll be regulated is very much influenced by your early environment and the type of rearing you have."

Even long after difficult events had passed stress was still having an effect on their bodies, added Prof Seth Pollak, a co-author of the study."That can affect their learning and their behaviour, and having a compromised immune system is going to affect these children's health," he said.

The study looked for high levels of the bodies own defences against the common, and usually latent, cold sore virus in 155 adolescents. Around two thirds of the population carry the virus, those with healthy immune systems are able to keep it mainly suppressed and rarely if ever suffer flare ups.

However, those whose immune systems have been depleted can have trouble fighting the bug. The findings, published in the Proceedings of the National Academy of Sciences journal, shows that those who had stressful childhoods had higher levels of cells the body makes to attack the virus, showing that their immune systems were compromised.