In recent years there have been many news stories connecting mental health and physical health. For a long time they were regarded as separate areas. But the brain is part of the body connected to blood supply, to nerves that interact with other parts such as the digestive system. The brain affects the rest of the body via electrical and chemical messages. There is feedback from the body to the brain also through sensations and chemicals. Physical health problems such as viral infections can affect your behaviour, thoughts and emotions. You may feel unmotivated, delirious, depressed.
Mental health problems increase the risk of physical problems. People with schizophrenia are twice as likely to die of heart diseases and three times as likely to die from respiratory disease according to 'Physical Health and Mental Health' from Mental Health Foundation, a British site. This may be for a number of reasons. So many schizophrenics smoke. Most schizophrenic people I've known have been heavy smokers. Still there are quite a few mentally ill people who never smoke. I once was in a group of mentally ill people and we were all surprised that no one in it smoked.