What is the Difference Between Infectious and Contagious

The main difference between infectious and contagious is that infectious diseases refer to diseases caused by pathogenic microorganisms like viruses, bacteria, and parasites whereas contagious diseases refer to diseases that spread from one organism to another, through direct or indirect contact.

Infectious and contagious are two adjectives we use to describe diseases. In general usage, it is possible to use these two words interchangeably since both infectious and contagious diseases disease-producing agents like viruses and bacteria cause both. However, there is a slight difference between infectious and contagious diseases, as not all infectious diseases are contagious.

Key Areas Covered

1. What Does Infectious Mean
     – Definition, Characteristics, Diseases
2. What Does Contagious Mean
    – Definition, Characteristics, Diseases
3. What is the Relationship Between Infectious and Contagious
     – Outline of Common Features
4. What is the Difference Between Infectious and Contagious
     – Comparison of Key Differences

Key Terms

Contagious, Diseases, Infectious

What Does Infectious Mean

Infectious diseases are diseases caused by disease-producing agents like viruses, bacteria, parasites and fungi. Common cold, flu, malaria, urinary tract infections, tuberculosis, chickenpox, influenza, dengue, SARS and AIDS are some examples of infectious diseases. Most of these diseases can spread directly or indirectly, through one organism to another.

Spread of Infectious Diseases

Direct contact – Coming into contact with an animal or person with the infection (through cough or sneeze, sexual contact, bite of a pet, from mother to unborn, etc.)

Indirect Contact – Disease-causing microorganisms may linger on an inanimate object like doorknobs, tabletops, mugs, shoes, etc.

Insect Vectors – Some pathogens have insect vectors like flea, mosquitos, or lice

Food Contamination – disease-causing germs may also contaminate food and drink, making people ill

Figure 1: Faecal-Oral Disease Transmission

Infectious diseases that spread from one person to another are actually called contagious diseases. However, not all infectious diseases are contagious. For example, food poisoning and Lyme disease are infectious diseases, but not contagious diseases as they do not spread from one person to another.

What Does Contagious Mean

Contagious means transmissible by direct or indirect contact with an infected person. Direct contact may involve direct physical contact like touching or kissing an infected person. Moreover, infectious microbes travel through the air when an infected person sneezes or coughs. Sometimes people also get contagious diseases by touching an object an infected person has used, for example, a straw, doorknob, etc.

Figure 2: Sneeze of an Infected Person can Spread the Contagious Disease to Others

Some Contagious Diseases

• Chickenpox
• Measles
• Flu
• Pertussis
• COVID-19
• Tuberculosis

Contagious diseases are also caused by disease-causing microorganisms. Therefore, all contagious diseases are infectious diseases. However, not all infectious diseases are contagious, as explained above.

Relationship Between Infectious and Contagious

• Disease-causing microorganisms like bacteria, parasites and viruses cause both infectious diseases and contagious diseases. 
• Therefore, we use these two terms interchangeably.
• All contagious diseases are infectious, but not all infectious diseases are contagious (ex: food poisoning, urinary tract infection, etc.)

Difference Between Infectious and Contagious

Definition

Infectious diseases refer to diseases caused by pathogenic microorganisms like viruses, bacteria, and parasites, whereas contagious diseases refer to diseases that spread from one organism to another, through direct or indirect contact.

Cause

Moreover, infectious diseases are caused by pathogenic microorganisms like viruses, bacteria, and parasites. They can spread through direct or indirect contact with infected people, food and water contamination or insect vectors. Contagious diseases, on the other hand, are spread through direct or indirect contact with infected people.

Examples

Common cold, flu, tuberculosis, chickenpox, measles, SARS, and COVID-19 are some examples of contagious and infectious diseases. Food poisoning, Lyme disease, and urinary tract infection are examples of diseases that are infectious, but not contagious.

Conclusion

The main difference between infectious and contagious is that infectious diseases refer to diseases caused by pathogenic microorganisms like viruses, bacteria, and parasites whereas contagious diseases refer to diseases that spread from one organism to another, through direct or indirect contact. All contagious diseases are infectious, but not all infectious diseases are contagious.

Image Courtesy:

1. “Sneeze” By James Gathany – CDC Public Health Image library ID 11162 (Public Domain) via Commons Wikimedia
2. “The F diagram showing pathways of faecal-oral disease transmission” By SuSanA Secretariat (CC BY 2.0) via Flickr