The Difference Between STI & STD STD stands for Sexually Transmitted Disease, whereas STI defied as Sexually Transmitted Infections. All STIs are also STDs. Essentially, the difference is between a Disease and an Infection . a disease is a disorder of structure or function in a human, animal, or plant, especially one that produces specific symptoms or that affects a specific location and is not simply a direct result of physical injury. There are four main types of disease: infectious diseases, deficiency diseases, hereditary diseases (including both genetic diseases and non-genetic hereditary diseases), and physiological diseases . Diseases can also be classified in other ways, such as communicable versus non-communicable diseases. an infections is t he invasion and multiplication of microorganisms such as bacteria, viruses, and parasites that are not normally present within the body. An infection may cause no sy