Viruses

Pathogens: Pathogens include both viruses and bacteria that cause diseases in living organisms.

Immune System: The immune system is the body's defense mechanism against pathogens. It recognizes foreign substances like viruses and mounts an immune response to eliminate them.

Vaccines: Vaccines contain weakened or inactive forms of a virus (or other pathogen) that stimulate an immune response without causing illness. They help prevent future infections by training the immune system to recognize specific pathogens.