This document discusses how infectious agents are transmitted from one host to another. It defines key terms like reservoir, which is any organism that harbors the infectious agent, and carrier, which is a person who can transmit the agent without showing symptoms. It describes the links in the infection chain, including the agent, portal of exit/entry, susceptible host, and mode of transmission. The main modes of transmission discussed are direct contact, indirect contact, droplet transmission, aerosol transmission, and vector-borne transmission through an intermediate host like an insect.

1. HOW INFECTIONS
SPREAD
1

2. Transmission of Infectious
Agents
Reservoir
•Any person, animal, soil or substance
in which an infectious agent normally
lives and multiplies
•Can be symptomatic, but usually
asymptomatic

3. Animal Reservoirs

4. Transmission of Infectious
Agents
Carrier
•A person who is a carrier of the
microorganism that may never
develop the disease and is able to
pass the microorganism to other
people

5. Links in the Infection Chain
Agent: the microbe
causing the infection

6.  Portal of exit – how agent leaves
reservoir

7. Portal of entry: access to new host
Susceptible host: a member of a
population who is at risk of becoming
infected by disease; depends on:
Health status
Immunity
Age
Nutrition

8. Mode of transmission
Method whereby the agent
reaches a new susceptible
host

9. Modes of Transmission
Direct contact
 No intermediary
 Direct contact with
infected blood or bodily
secretions
 EX: Touching infectious
lesion, sexual activity

10. Modes of Transmission
Indirect contact
 Involves intermediary object
or organism
 Contaminated hand or food
 Fomite—inanimate objects or
materials that are likely to carry
infection

11. Droplet transmission
 Respiratory or salivary
secretions are expelled from
infected individual
Aerosol transmission
 Involve small particles from
the respiratory tract
suspended in air and can
travel farther than droplets

12. Vector-borne
Insect or animal is an
intermediate host
Vector
an organism does not cause
disease itself spreads infection by
passing the pathogen from one
host to another; usually part of
the lifecycle of the pathogen

13. Vectors