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.

Viruses are obligate intracellular parasites, meaning they need a host cell in order to replicate. They do this by using their genetic material to take control of the reproductive machinery of the host cells to create new viruses. In the process, they cause host cell destruction, or lysis, and cause disease. Viruses can resist the immune system by evading recognition and hiding within host cells.

Fungi can cause superficial infections of the skin and mucous membranes, but they also can cause systemic infections. Some fungi secrete enzymes that damage host tissue. Other fungi can also use mechanisms to avoid being recognized by the immune system or produce substances that inactivate host immune defenses.

Risk factors for infections include several host characteristics like genetic predisposition, extremes of age, presence of chronic disease, malnutrition, and severe stress. Additionally, being under- or unimmunized or immunosuppressed, and having a break in physical barriers, such as the skin, also contribute to infection risk.

Now, the infectious process involves several steps. First, contact and transmission begins when an individual encounters a pathogen. The pathogen can be exogenous when it comes from external sources like contaminated food or another individual; or endogenous, which can happen if there’s an imbalance in the normal microbiome that causes an overgrowth of certain microorganisms.