Cystitis is a condition familiar to many women, and is usually easily treated with a course of antibiotics. It can become a problem if it tends to recur, and then investigation should be carried out to see if there is an underlying cause. If it occurs in young children, then it should be investigated immediately by a paediatrician. If it occurs in an adult, and is associated with passing blood, then it should also be investigated, by ultrasound and endoscopy of the bladder. It should probably also be investigated if it occurs in a man, but your own doctor will be best placed to advise on this. Infections in the kidney can be more serious, so don't hesitate to seek advice if you think this might be your problem.