Parents with stressful lives may be making their children as well as themselves vulnerable to illness, research suggests.
Bad blood may impact on children
A University of Rochester study, reported by New Scientist, found sickness levels were higher in children of anxious or depressed parents.
It also found links between stress and immune system activity in the children.
A UK scientist said children were "highly resilient", and urged parents not to worry about the findings.
It has long been known that stress can cause immune system changes that make a person more prone to infections and other illnesses.
However, the new study, first published in the journal Brain, Behavior and Immunity, is the first to provide evidence that the problem can be transmitted from parent to child.
The researchers, led by Dr Mary Caserta, followed the parents of 169 children over a three-year period, with the parents recording instances of childhood illness, before undergoing six-monthly psychiatric evaluations.
The total number of illnesses was significantly higher in the children of parents with higher levels of "emotional stress".
In addition, immune cells in the blood of the children was measured, and those with more stressed parents had raised levels.
The researchers conceded that allowing parents to measure the illness in their children could skew the results, as over-anxious parents were more likely to record higher levels.
However, they said that the findings still strongly suggested a connection between parental stress and children's health.
Dr David Jessop, a researcher in stress and health at Bristol University, said that longer-term studies were needed to measure both the immediate impact of the stress, and how children recovered in periods of less parental anxiety.
He said that it would be wrong for the research to worsen the plight of parents already suffering from stress.
"The 64,000 dollar question is, once this stress is alleviated, do the children's immune systems return to normal, or are they scarred for life?
"I believe that children are highly resilient, and their systems are sufficiently robust to cope with this.
"Although it is a good study, parents should not let it worry them unduly."