How Are Infectious Diseases Spread?

Infectious Disease by Definition:

In pure medical terms, an infection is the invasion and multiplication of a microbial agent in a human or living body. So, infectious disease means the illness and the deterioration in health which is caused by pathogens. When these microbial agents do not cause any harm to the health, it is referred to as subclinical infection. However, when they do affect the health, the condition is called infectious disease. Most of the infectious diseases (not all) are contagious. This means that they can spread from one person to another.

How Are Infectious Diseases Spread?

Infection Causing Agents:

The microbes that cause infection are known as agents. They can be divided into five main categories which include bacteria, virus, fungi, protozoa and helminthes (worms). These agents are living organisms that find the host on their own or are transmitted to a host body. Once they enter the host they start replicating. If tissues in the body react to the toxins produced by these pathogens, it effects and alters the health negatively. Not all microbes are harmful. In fact, our body houses a lot of bacteria and other microbes naturally. They are harmless and some are even useful and help our bodies in performing certain functions easily.

How are Infectious Diseases Spread:

As mentioned earlier, most of the infectious diseases are contagious. They are easily and quickly spread. When an infected person comes in contact directly or indirectly with a healthy or an uninfected person, these pathogens are transmitted to a new host body. In the new host they replicate and cause the same infectious disease as they did in their previous case. There are many ways in which they are transmitted and spread from one body to another. Have a look at the list below and read how are infectious diseases spread:

The main ways in which infectious diseases are usually spread are enlisted as follows:

  • They are spread through air.
  • From any kind of contact either direct or indirect between an infected and an unaffected person.
  • Saliva of the infected person/animal.
  • Blood or other bodily fluids.
  • Skin or mucous membrane in the body (Nose, mouth, genitals etc.)
  • By consuming contaminated food or water.
  • Urine or feces carrying infections.
  • Soiled, dirty and unhygienic objects.
  • Carried through sexual contact between partners.

These are the most common ways in which the pathogens spread infection. Let’s discuss them in detail for a better understanding of how are infectious diseases spread.

Air borne Pathogens:

Some pathogens are carried through the air particles. When an infected person coughs or sneezes, the pathogens are released into the air. These can be inhaled by another person and make them sick. The main diseases that are spread by air include chicken pox, common cold, diphtheria, flu, measles, meningitis, mumps, rubella, strep throat and tuberculosis, etc.


Some infections are spread by the direct or indirect contact with the infected person’s saliva. For example, this happens directly in the form of kissing or by sucking or sharing things among children. The diseases that are transmitted this way are hepatitis B, glandular fever, cytomegalovirus infection, etc.


Some diseases are spread by the urine of infected person coming in contact with a healthy individual through dirty hands or objects. One example of such disease which is spread through urine is cytomegalovirus (CMV) infection.


A number of infectious diseases are spread from a microscopic amount of feces of an infected person coming in contact with the un-infected person. Through soiled hands, objects, surface, food or water, those toxic microbes can carry forward or spread the infection. Hepatitis A, Meningitis, Giardiasis and Thrush are a few of the many diseases spread this way.

Skin or Mucous Membrane Contact:

Infection is spread easily by the direct contact of skin or mucous membrane, like that of nose, throat, genitals, etc. of an infected person with the skin of a healthy person. This can also be carried indirectly through other mediums, like objects, towels, clothes and sheets which are used by the infected person. Conjunctivitis, skin worms, chicken pox, hand foot and mouth disease, scabies, school sores and warts etc. are usually spread this way.

Sexually Transmitted Diseases:

These types of infections are usually carried from person to person through intercourse or any sexual contact, for instance, genital to genital, oral to genital, genital to anus, and so on. Hepatitis B, gonorrhea, genital wart, non-specific urethritis, HIV/AIDS, syphilis and pubic lice are some main infections spread this way.

Blood or Body Fluids:

These infections are spread when the blood or any bodily fluid of the host or the carrier reaches a non-carrier body and starts replicating. This happens through a used injection, needle pin or broken skin. The most common diseases spread this way include hepatitis B, hepatitis C, cytomegalovirus and human immunodeficiency virus aka HIV.

Food & Water Borne Diseases:

These are commonly spread through drinking contaminated water or soiled food. The toxins which are already present in the food or water reach a new host and start replicating and causing the disease. This is the most common way of spreading these infections. Botulism, cholera, shigella, salmonella, typhoid, yersinia and listeria are a few names of the many diseases spread through food and water contamination.

Direct or Indirect contact of an infected body with an uninfected person:

Most of the diseases are spread through indirect contact of the two bodies, the carrier and the Non-carrier. The environment, animals and objects play a major role in spreading the infection or disease. Some diseases spread by animals are cat scratch disease, hydatid disease, Q fever, rabies, toxoplasmosis and psittacosis, etc. The diseases spread by insects include malaria, dengue, Ross river infection and Barmah Forest virus infection to name a few. Tetanus, legionella infection and amoebic meningitis are spread through the contact of environment like soil or water.

Taking safety measures or precautions is mandatory to keep yourself protected from these infections. Getting routine vaccination, especially, that of your children is very important. Maintaining sanitary conditions and washing hands are the simplest ways to keep away from these contagious infections.

About the Author

Posted by: M. Isaac / Senior writer

A graduate in biological sciences and a PhD scholar (NCBA&E University, Lahore), M. Isaac combines his vast experience with a keen and critical eye to create practical and inherently engaging content on the human body. His background as a researcher and instructor at a secondary school enables him to best understand the needs of the beginner level learners and the amateur readers and educate them about how their body works, and how they can adopt a healthier lifestyle.

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