Pathogens are microorganisms that can cause disease in humans, animals, and plants. They include bacteria, viruses, fungi, and parasites. When these harmful organisms enter a host, they can disrupt normal bodily functions and lead to various illnesses. Pathogens can spread through different means, such as direct contact, contaminated food or water, and airborne transmission. The body's immune system works to fight off these invaders, but sometimes pathogens can overwhelm it, leading to infections. Understanding how pathogens operate is crucial for preventing and treating diseases.