Children with autism struggle to communicate with those around them
Caring for children with developmental problems such as autism or Down's syndrome can weaken parents' immune systems, research suggests.
Researchers at Birmingham University found they had a poorer immune response to a vaccine against pneumonia.
It appears that stress causes the immune system to function less efficiently, the team wrote in the journal Brain, Behavior, and Immunity.
Charities called for better support for parents struggling to cope.
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 pointed out.
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% of parents providing long-term care had an ineffective immune response, compared to 4% of the control group. At six months this had risen to 48% while the levels in the control group remained the same.
Study leader Stephen Gallagher said low levels of antibodies suggested parents' ability to fight infection was weaker: "This is a good indication that their immune systems are not functioning efficiently."
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."
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 the parents should be added to the list of vulnerable groups eligible for vaccinations such as the flu jab.
"We are continuing this work by looking at how sleep patterns affect the rates of infection in these parents," she added.
Amanda Batten, head of policy and campaigns at the National Autistic Society, said: "Carers often display great strength and resilience whilst coping with this complex disability, but many receive very little help and support, and are under considerable stress as a result.
"It is imperative that carers are given access to services such as short break schemes to help them before they reach a crisis point."
Carol Boys, chief executive of the Down's Syndrome Association, added that caring for a child with Down's syndrome can be very rewarding but for some families it can be an extremely stressful experience.
"I am not surprised at the results of this study. It just confirms what we already suspected might be one of the consequences of the constant stress that families have to endure."