The study conducted by researchers at Birmingham University found that the parents whose children were autistic had a poorer immune response to a vaccine against pneumonia.
According to the research team, led by Stephen Gallagher, stress is the culprit behind parents' weak immune system.
Previous work has shown negative effects in elderly people caring for a spouse - but this is the first time that a similar result has been seen in a younger, healthier group providing round-the-clock care, the researchers said.
In the research, a total of 60 parents received the pneumococcal vaccine as part of the study - half of whom had children with developmental disabilities.
Blood tests showed that those caring for a child with developmental disability had lower levels of antibodies to the vaccine than those whose children did not have such difficulties.
After one month, 20 percent of parents providing long-term care had an ineffective immune response, compared to 4 percent of the control group.
At six months this had risen to 48 percent while the levels in the control group remained the same.
Gallagher said low levels of antibodies suggested that parents' ability to fight infection was weaker.
"This is a good indication that their immune systems are not functioning efficiently," BBC quoted him, as saying.
Stress was likely to be responsible for the immune deficiency, he added.
"These parents are sometimes extremely stressed and what they need is appropriate help and training," he said.
Co-author Dr Anna Phillips said parents caring for these children are "incredibly dedicated" and not in a position to take time off.
"However, knowing the effects that providing round-the-clock care can have on their health may help raise awareness that these parents need help to manage their burden of care," she said.