Disease is a medical condition that affects one or more organ systems and usually produces specific symptoms. The symptoms vary with the disease and its underlying cause. Some diseases are localized (affecting only a small area of the body), while others are disseminated or systemic (affecting the whole body).

A disease is a harmful change in the structure, function or biochemistry of an organism. The term disease is most commonly used to refer to communicable diseases – those caused by pathogenic microorganisms such as viruses, bacteria, fungi or protozoa. However, it can also be applied to non-infectious diseases such as cancer or diabetes, which are not transmissible from person to person.

Infections can occur by direct contact with someone ill or with the fluids they discharge, such as a sneeze or cough. They can also be spread by insects such as mosquitoes, fleas and ticks that harbor germs or through contaminated food and water. Some diseases are hereditary and are passed from parents to their children.

It is important to distinguish between a disease and a disorder. The former has a distinct cause and clear-cut symptoms, while the latter does not. It is sometimes easy to confuse signs with symptoms, especially since the latter are only known to the individual who experiences them. For example, a bloodshot eye or swollen fingers may seem trivial to someone who knows that they are an indication of an underlying health problem such as an infection or blood disorder.