The scientific definition of a pathogen is an organism, such as a bacterium, virus, fungus, or parasite, that can cause disease in its host. Pathogens can invade the host's body, replicate, and disrupt normal biological functions, leading to illness. They may spread through various transmission methods, such as direct contact, airborne particles, or contaminated food and water. Understanding pathogens is essential for public health, disease prevention, and the development of effective treatments and vaccines.