The Infectious Process

The infectious process refers to the steps that occur when a pathogen, which is a disease-causing microorganism, enters the body, multiplies, spreads, and causes tissue damage.

Okay, so, the body has lines of defense to protect itself against pathogens. The first line of defense includes physical and chemical barriers that prevent entry of pathogens into the body, like intact skin and mucous membranes, and molecules like lysozymes found in body fluids like tears and mucus, that can break down the cell walls of certain pathogens. Another key aspect of defense is the microbiome, or the collection of microorganisms that normally live in and on the body. These microorganisms don’t normally cause disease, and they help protect against invading pathogens by mechanisms like physically occupying space and even producing substances that inhibit the colonization of pathogens.

If a pathogen gets past these barriers, it encounters the second line of defense, the inflammatory response, where immune cells like macrophages and neutrophils phagocytize, or ingest, bacteria and cellular debris. Other systems, like the complement system, also get activated, which assist in killing of pathogens. Both the first and second lines of defense are innate, meaning they’re natural, inborn defense mechanisms, and they’re non-specific, because they respond the same way to all pathogens.

Now, if these mechanisms are not able to stop the invasion of the pathogen, the third line of defense, adaptive immunity, kicks in. It involves cellular immunity primarily with T lymphocytes and B lymphocytes, and the formation of antibodies, which is also called humoral immunity. Adaptive immunity mechanisms attack the current infection, and it even has a memory; so, the next time the body is exposed to this same pathogen, it can start attacking it faster.

Alright, so, infections are caused by pathogens like bacteria, viruses, and fungi. Each of these organisms have their own characteristics that contribute to their pathogenicity, or their capacity to cause disease, and virulence, which is the degree of pathogenicity of a specific microorganism.

Bacteria have certain characteristics that help them cause infection in an individual who becomes the host to the pathogen. These include pili, which are small hair-like appendages that help them adhere to host cells; toxic secretions including exotoxins, endotoxins, and enzymes that damage host cells; capsules, or an outer covering that prevents their destruction by the immune system; and biofilm, a matrix they excrete to further protect themselves from destruction.