Communicable diseases may be transmitted from the reservoir or source of infection to a susceptible individual in different ways depending on the infectious agent, portal or entry and local ecological conditions.
Modes of transmission can be classified into direct transmission and indirect.
Direct Transmission includes:
- Direct contact
- Droplet infection
- Contact with soil
- Inoculation into skin or mucosa
- Trans-placental or vertical.
Indirect Transmission includes:
- Vehicle borne
- Vector borne
- Air borne
- Fomite borne
- Unclean hands and fingers.
- Direct Contact: Infection may be transmitted from skin to skin, mucosa to mucosa, mucosa to skin, e.g. STD, AIDS, leprosy, skin and eye infections.
- Droplet infection: Direct projection of spray of droplets of saliva and naso-pharyngeal secretions during coughing, sneezing, spitting, e.g. respiratory infections, cold, fever, tuberculosis etc.
- Contact with soil: Direct exposure of susceptible skin to soil contaminated with agent, e.g. hookworm, tetanus.
- Inoculation into skin or mucosa: Disease agents may be directly inoculated into the skin or mucosa, e.g. dog bite, contaminated needles, syringes.
- Trans-placental: Transmission of the agents through placenta, e.g. AIDS, hepatitis B, Syphilis.
Five Fs are involved in transmission - flies, fingers, fomites, food and fluid. It is possible only if the agent is capable of surviving out side the human environment and retains its basic properties till it enters new host.
- Vehicle – borne: This implies transmission of the agent through water, food, ice, blood, serum and biological products. Most common are food and water. Some agents develop in the vehicle while some others use it purely as a vehicle, e.g. typhoid fever, diarrhea, cholera, polio, malaria etc.
- Vector borne: Vector is defined as an arthropod, or any living carrier that transports an infectious agent to a susceptible individual.
Mechanical vector borne: The vector carries the agent on its body, may be feet or through its gastrointestinal tract passively excreting the vector without any multiplication.
Biological vector borne: The agent undergoes replication and / or development in the vector and hence requires incubation period.
Factor which influence the ability of vector to transmit disease are
- Host feeding preferences
- Survival rate of vectors in the environment
- Suitable environmental factors
Droplet nuclei: They are the type of particles implicated in the spread of airborne infection. Tiny particles which represent dried residue droplet. They may be formed by
- evaporation of droplets coughed or sneezed
- generated purposefully by atomizing devices
- formed accidentally in microbial laboratories, in abattoirs, rendering plants, autopsy rooms
Droplets remain floating in the air, some may retain virulence, others may lose infectivity and virulence, e.g. TB, influenza, chickenpox, measles, etc.
Dust: Larger particles expelled during talking, coughing, sneezing settle out on carpets, furniture, clothes, bedding etc. A variety of infecting agents are reported in hospital beds, wards, living rooms of houses. During dusting, sweeping, bed making the agents become part of air or air borne.
Fomite borne: Fomites are inanimate articles or substance (other than water or food), contaminated by the infectious discharges from a patient and capable of harboring and transferring agents to a healthy person. Fomites include soiled clothes, towels, pens, pencils, taps, surgical dressings etc. Examples of diseases transmitted by fomites include – diphtheria, typhoid fever, dysentery, eye and skin infections.
Fingers and hands: Fingers and hands are the most common agents transferring infections. Hands and fingers act as media of transfer of agents to food. Examples include infections, typhoid fever, dysentery, etc. Lack of hygiene is implicated by cleanliness of hands. Poor sanitation, hygiene and lack of cleanliness result in person to person transmission of the disease causing agents.