Symptoms are sensations in the body that only the person who is unwell can experience; for example, a headache, pain in the abdomen, blurred vision and nausea are all symptoms, because no one but the sufferer can experience them. Lay people, including children, can usually describe their symptoms accurately or report them if asked the right questions, but symptoms are subjective experiences that others cannot observe or verify.
By contrast, the signs of a disease are indicators of illness that other people can observe (e.g. a runny nose and frequent sneezing). Considering the symptoms and signs together may give enough information for a trained health care worker to make a diagnosis, i.e. identify the underlying cause of the illness and give it a definite name.