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. 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. Explore comprehensive guides on hundreds of common and rare diseases and conditions from the experts at Mayo Clinic. A surge of cases of the intestinal illness that causes diarrhea and fatigue has been detected in 34 states. Here's how what to do if you suspect you have it and how to avoid infection. 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. Symptoms of infectious diseases vary depending on the cause of the infection. But they often include fever and tiredness. Rest and home remedies may clear up mild infections. But other infections can be life-threatening, and treatment could involve hospital care. The meaning of ILLNESS is sickness. (Definition of illness from the Cambridge Advanced Learner's Dictionary & Thesaurus © Cambridge University Press) 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. A to Z list of common illnesses and conditions including their symptoms, causes and treatments.