Explore comprehensive guides on hundreds of common and rare diseases and conditions from the experts at Mayo Clinic. The meaning of ILLNESS is sickness. Viral infections are any illness you get from a virus (a small germ that uses your cells to reproduce). Examples include the flu, colds, COVID-19 and HPV. WebMD Symptom Checker is designed with a body map to help you understand what your medical symptoms could mean, and provide you with the trusted information you need to help make informed decisions. (Definition of illness from the Cambridge Advanced Learner's Dictionary & Thesaurus © Cambridge University Press) Illness represents a subjective experience of feeling unwell, often accompanied by suffering or discomfort. It signifies a deviation from an individual’s perceived normal state of health and well-being. A to Z list of common illnesses and conditions including their symptoms, causes and treatments. There are four main types of disease: infectious diseases, deficiency diseases, hereditary diseases, and physiological diseases. Diseases can also be classified in other ways, such as communicable versus non-communicable diseases. Is there a difference between a disease and an illness? Technically, yes. A disease is something a physician can diagnose, and an illness is something you feel. Find out what's going around near you right now — flu activity, COVID-19 levels, RSV, measles, and local health context for all 50 states and 499 U.S. cities.