Summary
Highlights
This video introduces the concept of mechanisms of pathogenicity, explaining how organisms cause disease once they have entered a human host. The goal is to provide a basic understanding of all the steps involved.
Pathogens must first find a way into or onto the body. The most common portals of entry are mucous membranes, particularly through the respiratory, digestive, and genitourinary systems. Other routes include breaks in the skin or the parenteral route, which bypasses skin and mucous membranes, often through bites or needle sticks.
Once inside, there must be a sufficient number of organisms, known as the infectious dose (ID50), to cause sickness. Adherence is also crucial; pathogens must attach to a specific location within the host to establish an infection, which is considered the true beginning of an infection.
After adhering, pathogens must contend with the host's immune system. They can evade detection and destruction through structural features like capsules, hiding inside host cells (intracellular pathogens), or by changing their surface proteins through antigenic variation to avoid immune recognition.
Once established and immune defenses are bypassed, pathogens cause damage to host cells. This can involve direct physical damage, the production of enzymes that break down tissues, stealing nutrients (like iron using siderophores), or the release of powerful toxins (endotoxins and exotoxins).
Finally, for the pathogen to continue its life cycle and spread, it must find a way out of the host. The portal of exit is typically the same as the portal of entry (e.g., respiratory system for entry and exit). This allows the pathogen to infect a new host and is crucial for its survival and transmission.