the immune system attacks bacteria / viruses once they have invaded the body
bacteria_can_cause_disease is pathogen. bacterial_agent is bacterium. bacteria is not one_cause_of_illness. plague_bacterium is bacterial_agent. pathogen is microorganism. bacteria is one_cause_of_illness. bacterial_infection is infection. bacterial_pneumonia is bacterial_infection. bacteria_can_cause_disease is not pathogen. pathogen is infectious_agent.