Mode of Transmission of Diseases

• The traditional epidemiologic triad model holds that infectious diseases result from the interaction of agent, host, and environment.
• More specifically, transmission occurs when the agent leaves its reservoir or host through a portal of exit, is conveyed by some mode of transmission, and enters through an appropriate portal of entry to infect a susceptible host. 
• An infectious agent may be transmitted from its natural reservoir to a susceptible host in different ways.
• There are different classifications for modes of transmission. 
• Diseases can be transmitted directly or indirectly.