Disease, Immunology, Medical Microbiology, Virology

Transmission of Diseases

Content hide 1 1. Airborne transmission 2 2. Contact Transmission 3 3. Vehicle Transmission 4 4. Vector-Borne Transmission 5 Carrier of Pathogen...

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Transmission of Disease

Maintain an infectious disease in a human population the pathogens must need to be transmitted from one host or source to another. In Biology the term transmission is refers to the passing of a pathogen from one individual to another individual which can cause a serious disease or infection. The transmission of disease is occurred by these following routs

1. Airborne transmission

In Airborne transmission of disease, the pathogens are remain suspended in the air and they can be travels over a meter or more from the source to the host. The pathogens are mainly transmitted through droplet nuclei or bioaerosol. When an infected animal or a person sneezes they released the pathogen into the air, now this pathogen can affect others if someone gets in contact with this environment.

Example: Pathogens of tuberculosis, chickenpox, measles, etc.

Airborne transmission of disease causing pathogen can be occurred by these two methods;

a. Aerosol

The suspended solid or liquid particles in the air are known as aerosol. The diameters of an aerosol vary between about 0.002 µm to about 100 µm. Aerosol can be a solid particle or a liquid particle or both. The pathogens can be transmitted by getting attached to the surface of these solid particles or aerosols.

b. Droplet nuclei

The remaining particles after evaporation of a droplet is known as a droplet nuclei. The size of a droplet nuclei varies between 1-5 µm. The droplets are mainly generated during coughing, sneezing, or talking. Droplet nuclei are too small and light that is why they can remain suspended in air for several hours. Thus, they may also infect a normal person entering a room that has been left by a patient long ago.

Transmission of Disease
Credit: James Gathany – CDC Public Health Image library ID 11162

2. Contact Transmission

Contact transmission means when a person coming together or touching of the source or Reserve of the pathogen and the host. Contact Transmission can be occurred by direct contact or indirect contact.

The Contact transmission of disease divided into two classes;

a. Direct Contact Transmission

Direct contact transmission occurs by direct body contact between two individuals. Direct contact transmission is divided into 2 groups;

  • Horizontal transmission

In this type in infected person can pass the pathogen by shaking hands, kissing, touching sores, or having sexual contact.

  • Vertical transmission

In this type, the pathogens are passed from parent to offspring in an egg or sperm, across the placenta, in breast milk, or the birth canal (as can happen with syphilis and gonorrhea).

b. Indirect Contact Transmission

Indirect contactor refers to the transmission of disease causing pathogen from the source to the host through an intermediary (Inanimate Objects). Some example of intermediate inanimate objects is a thermometer, bedding, clothes, drinking glass, needle, etc. Pseudomonas easily transmitted by this route.

Transmission of Disease
Image Source: www.markfranklinarts.co.uk

3. Vehicle Transmission

When inanimate material or objects are involved in pathogen transmission are called vehicle transmission. Vehicle transmission supports the multiple transmission of a pathogen but it does not support the reproduction of pathogen.

Example: Surgical Instruments, bedding etc. In Epidemiology this common vehicle is called fomites

4. Vector-Borne Transmission

When a living or animate material help in the transmission of disease causing pathogen are called vectors. Most of the vectors are arthropods (Insects, mites, flies) or vertebrates (Dog, Cat, bat).

Vector-Borne transmission is divided into two classes;

a. External Vector-borne transmission

In this type, the pathogen is carried out on the body surface of a vector. During the external vector-borne transmission, no growth of the pathogen occurs.

Example: Flies carrying shigella organisms on their feet from the physical source to a plate of food.

b. Internal Vector-borne transmission

In internal factor Bond transmission, the pathogens carried within the vector. Internal Vector-Borne transmission is subdivided into two categories;

  • Harbourage Transmission:

In harbourage transmission of disease, the pathogen does not undergo morphological or physiological changes within the vector.

Example: Agents of Plaque would be transferred by the rat to human.

  • Biological Transmission

In biological transmission of disease, the pathogen does go through a morphological or physiological change within the vector.

Example: Malaria parasite Plasmodium vivax transmitted by the mosquito from human to human.

Carrier of Pathogen

In the transmission of disease causing pathogen, a carrier also plays an important role. a carrier is an infected individual who is a potential source of infection for others. carrier plays an important role in the epidemiology of a disease. there are four types of carriers.

a. Active carrier

The inactive carrier is an individual who has over clinical cases of the disease

b. Convalescent Carrier

A convalescent carrier is an individual who has recovered from the infectious disease but continues to harbor a large number of the pathogen.

c. Healthy Carrier

A healthy carrier is an individual who harbors to the pathogen but is not get ill.

d. Incubatory Carrier

An incubatory carrier is an individual who is incubating the pathogen in a large number but is not it.

Convalescent carrier healthy carrier or incubatory carrier Mein Har Bar the pathogen for only a brief period and then they are called a casual carrier or an acute carrier or transient career.

If they harbor the pathogen for a long period then they are called chronic carriers.


  1. BODE SCIENCE CENTER, “Airborne droplet nuclei
  2. Regional radiative effects due to anthropogenic aerosols.
  3. Microbiology Principles and Explorations by Jacquelyn and Laura Black

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