Nearly half of all patients admitted to hospital have more than one health problem. The concept of co morbidity is not new. It has been around since the 1970s.

Depression is not the end of a one-way street. It can be the cause of other illnesses.

What is new, however, is that where more than one condition exists, our linear approach to a single diagnosis and a single treatment doesn’t quite work.

Take depression. Nearly one third of patients admitted to hospital suffer from depression. The depression recognised in these patients is not always a sequel to another health problem. Historically, we have believed that depression is the result of something else: a triggering factor such as another psychiatric problem or a genetic predisposition.

We now know that the route to depression is not a one-way street: rather than just being the result of another illness, depression can be the cause other illnesses. It is a two-way street.

One in six people with diabetes are likely to experience depression and depressed people are three times more likely to suffer subsequently from diabetes than the general population. Similarly,depression and physical frailty in later life have a bidirectional association: each of them can cause the other.

It not just depression or other reactive mental health problems that challenge current thinking about sequential diagnosis and what comes first. Bidirectional relationships have been found in a range of health conditions, for example periodontitis is related to a range and of comorbidities including cardiovascular disease, diabetes, rheumatoid arthritis, osteoporosis, Parkinson’s disease, Alzheimer’s disease, psoriasis, and respiratory infections.

Bidirectional causes of disease will alter the way we practice medicine. Rather than the old way, where relative severity and urgency is assigned to each problem and then each one is treated in series, we will have to deal with multiple problems all at once.

Sometimes this may result in uncomfortable decisions. For example, cancer is very serious but delaying treatment by one week probably doesn’t make any difference to the final outcome. Whereas delaying treatment for depression may be more likely to result in serious consequences beyond mental health.

Managing each disease as a distinct clinical entity rarely works to address the aggregate effects of interacting chronic conditions, so understanding if there is a clear cut causology becomes important.

For example, we know a lot about the direct relationship between obesity, diabetes and heart disease. Clinicians who deal with these patients are skilled in managing weight, lipids and sugar within a matrix of treatments. As more and more bidirectional associations are described integrated care rather than sequential care will become the treatment of the future.