The germ theory of disease eventually led to the development of certain types of vaccines. This theory explains that specific type or types of microorganisms, which are bacteria, virus, fungi, or protest species, cause some diseases. These organisms can enter the human body, and can even grow and reproduce, causing infection, inflammation to specific organs in the body, and others. One particular vaccine developed successfully through this understanding is the smallpox vaccine, which is an important vaccine up to this day.