Every pathogen has some reservoir outside the body of the human being, where it remains without causing disease.
It infects healthy persons through vectors, carriers or contact.
Transmission of Pathogens:
The pathogens of diseases are transmitted from the reservoir to a healthy person by two processes.
1. Direct transmission:
Direct transmission occurs through several ways.
(i) Direct Contact:
If a healthy person comes in direct contact with the patients or his discharged materials, the disease is transmitted.
(ii) Droplet Infection:
Droplets carrying infecting pathogens spread through air by coughing, sneezing, talking and spitting of the patient.
(iii) Contact with Soil:
Some saprophytic disease causing organisms remain inside soil. They enter the human being through injury.
(iv) Bite of an Animal:
Some diseases like rabies infect through bite of dogs.
(v) Trans placental Transmission:
Some diseases pass from mother to child through placenta.
2. Indirect Transmission:
Many pathogens are transmitted through some agents. It is known as indirect transmission.
(i) By Vectors:
Some insects, animals and human carrier spread diseases as carriers or vectors. For example, blood sucking insects, housefly, rodents act as vectors of diseases like malaria, cholera, plague etc.
(ii) Vehicle-borne:
Some diseases causing pathogens are transmitted through agencies like water, food, blood and ice etc.
(iii) Air-borne:
Diseases causing aerosols and dust are spreaded by air.
(iv) Fomite-borne:
Diseases are transmitted through contaminated articles like garments, toys, utensils, taps, door handles, syringes, surgical instruments etc.
(v) Unclean-hands:
By unclean hands and finger nails, some pathogens are transmitted.
Barrier to Invasion by Pathogens:
The pathogens produce diseases through invasion and produce toxins. The toxins and invasions were prevented by three ways in human being.
1. Mechanical Barrier:
The dry skin and hairs form a mechanical barrier. The hairs of nose, mucous, antibacterial substances form the first line of defence.
2. Phagocytes:
After passing the first line of defence, if the pathogens enter into the body, they are attacked by the phagocytic cells (of leucocytes). It is called as second line of defence.
3. Immunity:
It is the third line of defence. The antibodies formed in the body, inactivate the antigens of pathogens.