Speech and language therapists provide treatment for people who have difficulties in speaking and communication or swallowing disorders. They work in conjunction with other medical staff. The disorders they treat may have psychological or physical causes (which may be congenital or as a result of illness such as a stroke, Parkinson's disease or surgery for throat cancer, etc). Speech disorders can sometimes be resolved relatively easily but sometimes they are difficult to treat, and may require sessions with the therapist for long periods, often for some years. The work is carried out in schools, hospitals, health centres and sometimes in the patient's home.