Infectious agents are microorganisms that can cause diseases in humans, animals, and plants. These agents include bacteria, viruses, fungi, and parasites. They can invade a host's body, multiply, and disrupt normal functions, leading to illness. Infectious agents can spread through various means, such as direct contact, contaminated food or water, and airborne transmission. Understanding these agents is crucial for preventing and controlling infections, which is essential for public health and safety.