People with multiple health worries are at a higher risk of developing further illnesses than those with only one, a new study has found.

They were being disadvantaged by a health system principally designed to cater to individual conditions, the study's authors said.

Having two or more illnesses - or comorbidity as it's called - put patients at higher risk in three ways, the Serious and Continuing Illness Policy and Practice Study found.

First, those with multiple medical conditions were more likely to get sicker because they were eating poorly to begin with, exercising less or were stressed.

Having several illnesses also made it harder for patients to identify new symptoms, commonly choosing to dismiss it as a part of an existing condition.

Comorbidity reduced a person's ability to effectively manage their combined treatments.

Healthcare providers interviewed in the study said having multiple conditions deterred people from staying in rehabilitation or care because the system was not equipped to deal with comorbidity.

It proved Australia's health system was failing this hidden group of sufferers, said Tanisha Jowsey, one of the report's authors from the Australian National University.

"We need clearer guidelines about managing common combinations of illnesses, particularly when they have similar symptoms but different treatments," she said.

People with multiple illnesses and their carers needed practical information and skills so they could make good decisions in complex situations and seek appropriate help from health professionals.

"Otherwise, they will continue to miss the warning signs of major health problems," Ms Jowsey said.

A total 129 people were interviewed for the study, which has been published in the online journal, Australia and New Zealand Health Policy.