Organs are structures made up from different tissues, such as nervous tissue, muscle tissue, epithelial tissue and blood tissue (a type of connective tissue). These tissues, with their own specialized cells, work organizedly to make the organ function as it's supposed to do.
NOT EXACT DEFINITION: Diseases that can be passed from one living organism to the other. Two examples are flu and several STIs (such as herpes). Sorry I couldn’t think of any more :)
