Infection, Infectious Diseases, and Epidemiology: Core Concepts and Mechanisms

Infection, Infectious Diseases, and Epidemiology

Learning Objectives

• Identify types of symbiosis between microbes and humans.

• Define normal microbiota, including resident and transient forms.

• Explain opportunistic pathogens and conditions enabling disease.

• Describe reservoirs of infection and their types.

• Summarize major modes of infectious disease transmission.

• Describe major portals of entry and the parenteral route.

Symbiotic Relationships Between Microbes and Humans

Types of Symbiosis

Symbiosis refers to the close association between two different species living together. In microbiology, microbes and humans interact in several symbiotic relationships:

• Mutualism: Both organisms benefit. Example: Colon microbes aid digestion and receive nutrients.

• Commensalism: One organism benefits, the other is unaffected. Example: Skin microbes utilize nutrients from skin secretions without affecting the host.

• Parasitism: One organism benefits at the expense of the other. Example: Disease-causing microbes such as Mycobacterium tuberculosis.

Normal Microbiota

Definition and Types

Normal microbiota are populations of microorganisms that colonize the body's surfaces (internal and external) without typically causing disease.

• Resident microbiota: Permanent inhabitants, established during the first year of life.

• Transient microbiota: Temporary inhabitants, present for short periods due to elimination by the immune system, physical/chemical changes, or competition (microbial antagonism) with resident microbiota.

Opportunistic Pathogens

Definition and Conditions

Opportunistic pathogens are microbes that cause disease only under certain conditions, such as:

• Introduction into an unusual site (e.g., Escherichia coli from gut to urethra causing UTI).

• Immune suppression (e.g., AIDS patients).

• Changes in resident microbiota (e.g., loss of microbial antagonism due to long-term antibiotic use leading to yeast infection).

Reservoirs of Infectious Diseases of Humans

Types of Reservoirs

Reservoirs are sites where pathogens are maintained as sources of infection. In humans, three main types exist:

• Animal reservoirs: Pathogens acquired through direct contact with animals/waste, eating animals, or bloodsucking arthropods.

• Human carriers: Asymptomatic infected individuals who can transmit pathogens to others.

• Nonliving reservoirs: Soil, water, and food can harbor infectious agents.

Modes of Infectious Disease Transmission

Contact Transmission

• Direct contact transmission: Physical contact between hosts (e.g., touching, kissing).

• Indirect contact transmission: Spread via fomites (inanimate objects such as doorknobs, utensils).

• Droplet transmission: Pathogens in droplets travel short distances (e.g., exhaling, coughing, sneezing).

Vehicle Transmission

• Airborne transmission: Pathogens travel more than 1 meter via aerosols (e.g., sneezing, coughing, air-conditioning systems).

• Waterborne transmission: Ingestion of contaminated water, leading to gastrointestinal diseases and fecal-oral infections.

• Foodborne transmission: Ingestion of pathogens in or on foods, especially inadequately processed, cooked, or refrigerated foods; foods may be contaminated with feces.

Vector Transmission

• Mechanical vectors: Passively transmit pathogens on their body to new hosts (e.g., flies transferring pathogens from fecal matter to food).

• Biological vectors: Biting insects (e.g., mosquitoes) transmit pathogens through their bite, often involving part of the pathogen's life cycle within the vector.

Portals of Entry

Major Portals

Portals of entry are sites where pathogens enter the body. The three major portals are:

• Skin: Acts as a barrier to most microbes. Entry occurs via hair follicle openings and gland ducts.

• Mucous membranes: Line cavities open to the outside, including:

  • Conjunctiva (eyelids and eyeballs)

  • Respiratory tract (inhalation of droplets, aerosols, dust particles)

  • Gastrointestinal tract (ingestion of contaminated food and water)

  • Genitourinary tract (reproductive and urinary systems)

• Placenta: Connection to the female; some pathogens can cross, causing spontaneous abortion, birth defects, or premature birth.

Table: Pathogens Crossing the Placenta

Pathogen	Condition in Adult	Effect on Embryo or Fetus
Toxoplasma gondii (Protozoan)	Toxoplasmosis	Abortion, epilepsy, encephalitis, microcephaly, mental retardation, blindness, anemia, jaundice
Treponema pallidum (Bacteria)	Syphilis	Abortion, multiorgan birth defects, syphilis
Listeria monocytogenes (Bacteria)	Listeriosis	Granulomatosis infantiseptica, death
Cytomegalovirus (DNA virus)	Cytomegalovirus infection	Deafness, microcephaly, mental retardation
Rubella virus (RNA virus)	German measles	Severe birth defects or death
HIV (RNA virus)	AIDS	Immunosuppression (AIDS)

The Parenteral Route

The parenteral route is not a major portal of entry but involves direct deposition of pathogens into tissues beneath the skin or mucous membranes, or into the blood. This can occur via puncture, injection, bites, cuts, or wounds.

• Examples: Needle sticks, insect bites, surgical procedures.

Additional info: The above notes expand on the basic definitions and mechanisms, providing context and examples for each concept. The table on placental transmission is reconstructed and summarized for clarity.