Generally, the longer you’ve had a particular illness and the more severe it is, the longer it will take to treat. For example, someone suffering from the common cold or food poisoning should only need treatment for a week or two, while someone with a 20 year history of psoriasis may need a 6 month, or more, treatment course. Diseases such as Hepatitis C, Parkinson’s, or rheumatoid arthritis, for example, may require ongoing low-dosage treatment to maintain health despite the illness.