Pathogens like viruses and bacteria reproduce rapidly within the body's cells which causes damage. Bacteria particularly may produce toxins which make you feel ill.White blood cells ingest bacteria or virally infected cells by phagocytosis to destroy them. They can also produce antibodies to destroy particular pathogens and antitoxins to counteract any toxins produced by the pathogen. Memory cells can form which then confer immunity to that pathogen by creating specific antibodies and remaining in the system.