Contagious vs. Infectious: Differences and Correct Use

Understanding the use of the terms “contagious” vs. “infectious” is essential when writing or speaking about health topics like epidemics and pandemics. You need to use these terms correctly to make yourself understood, but first you need to know what they mean.

contagious vs infectious contagious vs infectious

What Does “Infectious” Mean?

Infectious” is an adjective that refers to something or someone that can transmit viruses, bacteria, or other germs to people or animals. The thing described as “infectious” is likely to spread infection. A germ can be described as “infectious,” and so can something carrying a germ. Here are a few examples to help:

  • The chicken pox virus is highly infectious, so there is a vaccine to help prevent its spread.
  • Children must be kept home from school for at least 24 hours after their fever subsides so they are no longer infectious to others.
  • There are a variety of infectious diseases in the world, including influenza, coronavirus, and mononucleosis.

What Does “Contagious” Mean?

Something that is infectious can also be contagious. The word “contagious” is also an adjective, but it describes a specific type of infectious spread: person to person. A contagious disease is one that someone can “catch” from another person. These examples will show you how to use it properly:

  • Because you can’t catch asthma from someone else, it isn’t a contagious illness.
  • Sally had to stay home from school because her strep throat was contagious.
  • Tom wasn’t sure if his cough was contagious, so he decided to self-isolate to keep from spreading it to others.

Both Terms Can Be Used Figuratively

Although “infectious” and “contagious” have clear scientific and medical meanings, they can also be used figuratively. This is perfectly correct, and offers a vivid way to describe something that can be spread from person to person or from other thing to another thing.

In general, “contagious” has a slightly less positive connotation than “infectious” when you are using them figuratively. The following examples will help you use the terms this way:

  • He had an infectious laugh. Whenever anyone heard him, it was impossible to keep from laughing right along.
  • In a crisis, fear can be contagious. Leaders need to remain calm in order to keep others from panicking.
  • No one could feel sad watching the children open their gifts. Their joy was infectious.

“Contagious” vs. “Infectious”: Important Differences

Because these words overlap in their meaning, it can be difficult to know which one you want to use. The key is clearly understanding where they differ.

Who (or What) Has the Germ

One key difference between these terms is who or what is carrying the disease:

  • To use the word “contagious,” a person must have the germ or the germ must be one that is carried by people.
  • To use the word “infectious,” the germ can be carried by anyone or anything.

How the Germ Is Transmitted

Another important difference is how the disease is transmitted:

  • If you use “infectious,” the germ can be transmitted any way.
  • If you use “contagious,” it must be transmitted from person to person.

Quick Reference Guide With Examples

This quick reference guide has some examples of diseases, their method of transmission, and whether they are infectious or contagious.

Disease

Method of Transmission

Infectious or Contagious?

Lyme disease

Tick bites to humans or animals

Infectious but not contagious

Influenza

Person to person

Both infectious and contagious

Common cold

Person to person

Both infectious and contagious

Coronavirus COVID-19

Person to person

Both infectious and contagious

Ear infection

Bacteria colonizing fluid in the ear

Infectious but not contagious

Malaria

Mosquito bites to humans

Infectious but not contagious

How to Know Which Term Is Correct

Ultimately, like many other common word choice and grammar mistakes, you need a trick to remember whether “contagious” or “infectious” is the correct word. Remember, you can always use “infectious,” but you can only use “contagious” if the disease is spread from person to person. If you keep this in mind, you’ll always choose the right word.

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