1. Susceptible macroorganism (host)

2. Potentially harmful infectious agent (microbe)

3. Environmental conditions.

1. Environmental conditions

The role of the environment for the infectious process developing is the following:

• It affects the host defense

• It influences the microbial virulence

• It may be the source and reservoir of infection

• It provides the mechanisms and routs of microbial transmission

• Mechanisms of Transmission

Most of the infectious diseases are communicable, that is they can spread from one host to another. Direct transmission is the immediate transfer of the infectious agent from the reservoir (source of infection) to a new host with no intervening intermediary.

For diseases with human reservoirs, direct transmission may be vertical and horizontal. Vertical transmission is the spread of disease from parent to offspring by an infected sperm or egg or by passage of pathogens across the placenta during fetal development. Congenital infection is an infection that is acquired in utero and present in birth. Horizontal transmission is the spread of disease from person to person within a group (population).

Indirect transmission occurs when the infectious agent is transferred by an intermediary – a vehicle, a vector, or a contaminated (rather than infected) person.

A vehicle is a nonliving material capable of transmitting infectious agent (for example, bedding, clothing, cooking and eating utensils, surgical instruments, contaminated air, food, and water, soil, contaminated drugs, blood, discharges of infected persons, etc.). Living organisms that are intermediaries in disease transmission are called vectors. The vectors may be mechanical (they pick up microbes on their feet or other body parts, e.g. flies often gives microbes a free ride from feces to food), or biological (they are infected, not merely contaminated. Pathogens multiply within these vectors). Biological vectors may introduce the microbe by biting, or by depositing their contaminated feces or other excretions on the skin. (For example, mosquitoes, ticks, fleas, flies, lice, etc.).

Mechanisms of infectious diseases transmission are shown in Table 11-1.

• Portals of Entry and Exit

‘Portals of entry’ is a site that provides access to tissues where environmental and nutritional conditions are conductive to establishing infection and where local defense mechanisms fail to subdue the pathogen.

‘Portals of exit’ is the way of microbe’s escaping from the infected person. Most pathogens escape from the infected host though the same portal they used to enter the body. Nonetheless, additional portals of exit may develop.

Table 11-1

MECHANISMS OF INFECTIOUS DISEASES TRANSMISSION

Mechanisms (routes) of transmission	Modes of transmission	Vehicles/ vectors	Portals of entry	Infectious diseases
FECAL-ORAL ROUTE	Alimentary	Food	Gastro-intestinal tract	Enteric bacterial and viral infections (cholera, dysentery, typhoid fever, hepatitis A and E, etc.)
	Via water	Water
	Via contaminated objects	Dirty hands, cooking and eating utensils, mechanical vectors, etc.
TRANSMISSION VIA BLOOD	Direct (hemotransfusion, transplacentally, etc.)	Contaminated blood	Blood	AIDS, hepatitis B,C,D, malaria, plague, etc.
	Indirect	Surgical instruments, needles, syringes, biological vectors (insects), etc.
AEROGENIC ROUTE	Via respiratory droplets (coughing, sneezing, etc.)	Respiratory droplets	Respiratory tract	Influenza, common colds, meningitis, legionelliosis, diphtheria, tuberculosis, etc.
	Via dust particles	Contaminated dust particles
	Via aerosol	Contaminated aerosols
CONTACT ROUTE	Direct contact (touching, kissing, sexual contact, etc.)	Skin and mucous surfaces, infected sperm and saliva	Skin, mucosa, hair, nails, genitourinary tract	Gonorrhea, syphilis, gas gangrene, rabies, inflammatory diseases, etc.
	Indirect contact	Contaminated objects that contact skin/ mucosa/ wounds