INTRODUCTION

• Definition: Communicable diseases are illnesses caused by infectious agents or their toxic products, transmitted from an infected source to a susceptible host.

• Epidemiologic Triad:

  • Agent (bacteria, virus, parasite, fungi)

  • Host (susceptibility influenced by immunity, nutrition, genetics)

  • Environment (conditions favoring transmission, e.g., sanitation, climate).

• Chain of Infection: Requires reservoir, portal of exit, mode of transmission, portal of entry, and a susceptible host.

• Reservoirs: Human (cases, carriers), animals, and environment (soil, water, plants).

• Agent Factors: Pathogenicity, virulence, invasiveness, toxigenicity, viability, antigenic power, ease of transmission.

• Modes of Transmission:

  • Direct: contact, droplets

  • Indirect: airborne, vehicles (food, water, fomites), vectors (insects).

• Incubation Period: Time between entry of organism and appearance of symptoms—important for quarantine, surveillance, and prevention.

• Immunity:

  • Natural (skin, mucosa, gastric acidity).

  • Acquired: Passive (maternal antibodies, immunoglobulins), Active (infection, vaccination).

• Control & Prevention:

  • Targeting agent (sterilization, treatment).

  • Targeting reservoir (case finding, carrier control, animal management).

  • Breaking transmission (isolation, handwashing, sanitation, vector control).

  • Protecting host (nutrition, health education, immunization, chemoprophylaxis).

• Community Protection: Herd immunity develops when a large proportion of the community is resistant, reducing transmission to susceptible individuals.

• Applied Example: MERS-CoV illustrates the importance of understanding reservoir, transmission, incubation, and preventive measures in controlling new infectious threats.