We have artificial and we have natural and each one of these can be split into active and passive. So basically, during the process of vaccination, during the process of immunization, when we take a vaccine and we inject it into that organism, into that individual, we inject an inoculated form of that pathogen. And what that does is it gives us active immunity by not having to have to experience the same exact detrimental effect that we would naturally experience if we are infected in a natural way. Now, in this process, we gain memory cells. If we examine the natural process, we have to get the actual infection from that pathogen. We have to be infected by that pathogen to gain the same exact response to basically form those same memory cells that contain the antibody.