Frederic Griffiths experiments involved two different strains of a bacteria that causes pneumonia in mammals: a disease causing strain, called the smooth strain, and a harmless strain, called the rough strain. As expected, when healthy mice were injected with the smooth strain they died, and when there were injected with the rough strain they lived. When the smooth strain bacteria were killed with heat and injected into healthy mice, the mice were fine. However, when heat-killed smooth strain bacteria and rough strain bacteria were injected together into healthy mice, the mice died. The healthy mice injected with heat-killed smooth strain bacteria were