Pathogenic microorganisms, such as bacteria, viruses, fungi, and protozoa, can cause infections in humans and other organisms. Pathogens cause infections through a chain of events. These infectious agents enter a susceptible host through a portal of entry. The skin and the respiratory tract are examples of portals of entry. After entry, the infectious agents grow and reproduce in the host. The survival of infectious agents depends on how efficiently they can spread and infect other hosts. They spread through various modes of transmissions, such as physical contact, air, and food. Infectious agents eventually leave the host through a portal of exit, such as urine, blood, saliva, stool, and so on. Sepsis occurs when bacteria and their toxins are present in the bloodstream.