A disease is a deviation from your body’s normal state of health. It may cause pain, discomfort, loss of function or even death. Disease can be caused by infections, genetic conditions or a variety of other factors. Sometimes the immune system fights these threats quietly and you are not even aware that anything is wrong, but other times it fights harder and produces clear signals that something has gone wrong.
Health is a state of delicate physiological balance (homeostasis) in terms of chemical, physical and functional processes, maintained by a complex set of control mechanisms that are not fully understood. Disease is the consequence of a breakdown of these control mechanisms.
Signs are characteristics of a disease state that can be noticed by people other than the person affected, such as a fever, rash, swollen ankle or vomiting. These are often described as symptoms, though there is a distinction between them: Symptoms are subjective qualities that a person feels. Signs are objective qualities that can be observed by others, such as a high blood pressure or low pulse rate.
Infectious diseases are those caused by germs that can spread from one person to another through direct contact. Examples of infectious diseases include the common cold, HIV and malaria. Non-infectious diseases are those that are not caused by germs, such as heart disease and cancer. These can be classified further into categories such as progressive or non-progressive, and as communicable or non-communicable.