Diseased means affected by or suffering from a disease. When something is diseased, it's sick or unhealthy because of an illness or infection. A diseased tree might have bark that's rotting and falling off, leaves turning brown before autumn, or fungus growing on its trunk. A diseased organ in the body isn't working properly because illness has damaged it.

The word describes serious, ongoing illness rather than temporary sickness. You wouldn't say someone has a diseased throat because they caught a cold, but you might describe lungs as diseased if they've been seriously damaged by an infection or illness. Scientists study diseased tissue under microscopes to understand what went wrong and how to fix it.

Sometimes people use diseased metaphorically to describe something that's corrupted or unhealthy in a non-medical way, like calling a failing company “diseased” or describing a broken friendship as diseased. But the core meaning always suggests something once healthy that has become damaged by sickness or decay.