A pathogen is a microorganism that can cause disease in its host. This includes various types of organisms such as bacteria, viruses, fungi, and parasites. Pathogens can invade the body, multiply, and disrupt normal bodily functions, leading to illness. They can be transmitted through different means, including direct contact, contaminated food or water, and airborne particles. The immune system is the body's defense mechanism against pathogens. It identifies and attacks these harmful invaders to protect overall health. Vaccines are one way to help the immune system recognize and fight specific pathogens, reducing the risk of infection and disease, such as those caused by influenza or COVID-19.