The Magic School Bus S01 E03 "Inside Ralphie"
The class needs Ralphie to give them a story idea for broadcast day, but Ralphie is sick. His mother (Tyne Daly), a doctor, refuses to allow him to go to school. When Ms. Frizzle says that there is an amazing battle going on inside him, the class decides broadcast day should be about whatever is making Ralphie sick. After shrinking and going into Ralphie's veins through a cut, the class travels to Ralphie's sore throat to see what is making him sick. Then, after they forget to stay out of the crossfire, his white blood cells see the bus as a threat and attempts to devour it.