Co-occurring means they are happening at the same time. The most important thing to remember here is mental illness and addiction do not happen the same for everyone. Statistically–and this just means slightly more commonly than not–mental illness is present in someone who becomes addicted to a drug or substance. This does not mean mental illness “comes first,” just that, as far as co-occurring disorders are concerned, many people who are dealing with addiction are also dealing with a mental illness.