Introduction to the Infectious Process and Disease Transmission

Comprehensive vocabulary covers terms related to the infectious process, transmission modes, host defenses, and microbial virulence factors.

Infection

The entrance and multiplication of a microorganism in or on a host.

Infectious disease

An infection with functional and structural harm to the host that is usually accompanied by signs and symptoms.

Colonization

The presence and multiplication of new microorganisms that may cause an infectious disease or be eliminated by host defenses.

Pathogen

A microbe capable of causing a disease.

Pathogenicity

The ability to cause a disease by overcoming host’s defenses.

Pathogenesis

The steps or mechanisms involved in the development of a disease; the disease process.

Virulence

The degree of pathogenicity, often measured quantitatively by the number of organisms required to cause disease.

$50\%$ of lethal dose ($LD_{50}$)

The number of organisms needed to kill half of the hosts/population.

$50\%$ of infectious dose ($ID_{50}$)

The number of organisms needed to cause infection in half the hosts/population.

Nosocomial infection

An infection acquired in the hospital or other health care settings where the causative agent was not present or incubating at the time of entry.

Community-acquired infection

An infection where the causative agent is present or incubating at the time of admission into a health care facility.

Endogenous infection

An infection resulting from organisms that are part of the patient’s normal flora.

Exogenous infection

An infection resulting from organisms from external sources.

True pathogen

A microorganism with the ability to infect a host with a healthy immune system as well as those in an immunosuppressed state.

Opportunistic pathogen

An agent that attacks a debilitated host but presents no danger to a healthy individual.

Resident microbial flora

Microorganisms that colonize an anatomical area for months or years; also known as usual or indigenous flora.

Transient flora

Microorganisms that are only temporarily present at an anatomical site.

Asymptomatic carriage

A state where an individual maintains a reservoir for an organism and can serve as a source for others but does not have the infectious disease themselves.

Reservoir

The environment or place of origin of the infecting agent, such as humans, animals, water, food, air, or soil.

Vector

A living entity that acts as an intervening agent to transmit a microorganism from a reservoir to a host.

Fomite

A non-living entity or inanimate object, such as medical instruments or doorknobs, that can transmit infection.

Congenital/Vertical transmission

Transmission of a pathogen across the placenta or during passage through the birth canal.

Dolor, Calor, Rubor, and Tumor

The local signs of infection: Pain, Heat, Redness, and Swelling.

Incidence

The number of people in a population who develop a disease during a particular period of time, referring only to new cases.

Prevalence

The number of people in a population who develop a disease at a specified time, regardless of when it first appeared (includes old and new cases).

Herd immunity

A condition where many immune people in a community act as barriers to the spread of infectious agents, reducing disease occurrence.

Sporadic disease

A disease that occurs only occasionally, such as Typhoid fever.

Endemic disease

A disease that is constantly present in a population, such as the common cold.

Epidemic disease

A disease acquired by many people in a given area in a relatively short period of time, such as Influenza.

Pandemic disease

A disease that occurs worldwide, such as AIDS.

Latent infection

An infection where the causative agent remains inactive for a time then becomes active, such as Shingles.

Septicemia

A systemic condition commonly known as blood poisoning resulting from the multiplication of pathogens in the blood.

Incubation stage

The time interval between initial infection and the first appearance of any signs or symptoms.

Prodromal stage

The phase characterized by the appearance of first, mild, and often vague signs and symptoms like general aches and malaise.

Innate/Natural Immunity

Non-specific defenses present at birth that involve the first and second lines of defense and do not have memory.

Adaptive/Acquired Immunity

Specific defense involving the third line of defense (lymphocytes/antibodies) that is customized and possesses memory.

Opsonization

The process of coating a bacterium with antibodies or complement components to enhance phagocytosis.

Lysozyme

A low molecular weight enzyme found in tears, saliva, and sweat that lyses bacterial cell walls by hydrolyzing the peptidoglycan layer.

B cells

Lymphocytes that originate and develop in the bone marrow and are responsible for humoral immunity by producing antibodies.

T cells

Lymphocytes that originate in the bone marrow but mature in the thymus, responsible for cell-mediated immunity.

IgM

The first antibody produced during an infection (primary immune response) and is most active at $20-25^\circ\text{C}$.

IgG

The major immunoglobulin in normal serum involved in the secondary (anamnestic) response; it can cross the placenta to provide passive immunity to newborns.

Actively acquired immunity

Resistance achieved by contact with an antigen, causing the host to produce its own antibodies (e.g., through active infection or immunization).

Passively acquired immunity

Temporary resistance achieved by the administration of preformed antibodies, where the host does not produce its own (e.g., breastmilk or antitoxins).

Endotoxins

Lipopolysaccharides (Lipid A) released by Gram (-) bacteria when they are destroyed; they are heat-stable and cause non-specific effects like fever and shock.

Exotoxins

Poisonous polypeptides released by living Gram (+) or Gram (-) bacteria that are heat-labile and have specific effects on host cells.

Hyaluronidase

An exoenzyme known as the 'spreading factor' that breaks down the hyaluronic acid holding tissue cells together.

Neurotoxins

The most potent among exotoxins, affecting the central nervous system, such as Tetanospasmin or Botulinal toxin.