The Human Immune System Why do you get sick? How do you get better? A disease is any that affects normal of the body. Some diseases are , or are inherited, others are caused by materials in the , but most are caused by such as bacteria, viruses, and fungi. Pathogens are “sickness-makers.” With pathogens all around us, the system works like a protection system. Some of these defenses are simple keeping all invaders out, while others work like a security guard, trained to spot and pathogens who have slipped past the barrier.