“Contagious” vs. “Infectious”: The Difference Can Be Important

by John Kelly, Senior Research Editor at Dictionary.com

Whether it’s flu season, chickenpox at your kid’s school, concerns about measles in your town, or the coronavirus pandemic, the words contagious and infectious often come around in news and social media, in casual conversations and government communications.

While these two terms get used interchangeably, knowing the difference between them can, in some cases, be life-saving.

To bring you clarity and comfort, we’re diving deep into these—and many other—terms that come up within the topic of disease.

Here’s the big takeaway. All contagious diseases are infectious, but not all infectious diseases are contagious. Contagious diseases are infectious diseases that are easily spread through contact with other people.

But how did we get to this distinction? Let’s begin by zooming out from contagious and infectious to look at a bigger picture of disease. Stay with us.

For health, safety, and medical emergencies or updates on the novel coronavirus pandemic, please visit the CDC (Centers for Disease Control and Prevention) and WHO (World Health Organization). For more words related to the coronavirus, see our glossary. And for more important distinctions between confusing words related to the coronavirus, see our articles pandemic vs. epidemic, quarantine vs. isolation, respirator vs. ventilator, and virus vs. bacteria.

What is the difference between communicable vs. noncommunicable?

There are many different ways to classify diseases. One broad classification is whether the disease is communicable or noncommunicable.

Let’s start with noncommunicable diseases, sometimes abbreviated as NCDs. Noncommunicable literally means “not communicable.”

Yes, we know it might seem more logical to start by defining what something is as opposed to what it is not. But hear us out: we think this approach can help make these challenging terms and concepts a little easier to follow.

So, a noncommunicable disease isn’t communicable. That means these diseases are not transmitted through contact with an infected or afflicted person. Noncommunicable diseases are, instead, caused by various genetic, physiological, environmental, and behavioral factors.

The World Health Organization (WHO) identifies four main types of noncommunicable diseases:

According to the WHO, these diseases account for 71% (41 million) of all deaths globally each year.

Because these diseases are noncommunicable, a person with cancer or diabetes can’t spread it to someone else by touching them, for instance, or through the air or water.

But a communicable disease, in contrast, is one that can be spread from one organism to another. This includes the spread from person to person, or (when it is a zoonotic disease) animal to humans. Communicable diseases claim millions of lives each year, though considerably fewer than their noncommunicable counterparts.

OK, word break. What’s up with the word communicable? Another term for spread is communicate, hence communicable (capable of being communicated). Most of us probably use the word communicate for the various ways we share thoughts, feelings, information, and so on through talking and writing. But in medical and scientific contexts, communicate can mean “to give to another, impart, transmit,” as a disease. Communicable diseases are also referred to as transmissible.

Still with us so far?

So, communicable diseases can be spread. But, how such a disease is spread is at the heart of the technical difference between the terms contagious and infectious.

What does infectious mean?

To drill down, it helps to begin with the term infectious.

Infectious means “communicable by or causing infection.” Hey, we don’t want to take any terms for granted here, so let’s break this down some more. Infection refers to “the process or state of being infected with a disease.” And to infect means “to affect or contaminate someone or something with disease-producing germs.”

So, with infectious diseases, it’s all about spreading germs. Let’s get even more technical.

An infectious disease is caused by pathogenic microbial agents, such as viruses, bacteria, or other microorganisms. Pathogenic means “disease-producing.” Microbial refers to microbes, or microorganisms that can produce disease. Viruses are sometimes referred to as microbes because, like microbes, they are microscopic and can cause disease.

Put simply, an infectious disease happens when bad germs get into the body in some way, spread, and make you sick by affecting the way your body normally works.

Examples of infectious diseases:

  • chickenpox
  • cholera
  • common cold
  • coronavirus, including COVID-19
  • Ebola
  • flu (influenza)
  • hantavirus
  • hepatitis A & B
  • HIV/AIDS
  • Lyme disease
  • malaria
  • measles
  • meningitis
  • MRSA
  • polio
  • pneumonia
  • smallpox
  • STDs (sexually transmitted diseases)
  • tuberculosis
  • West Nile virus
  • Zika

Some of these diseases are common, like the cold and flu. Others are rare or have been effectively eliminated in many parts of the world.

Facts about the word infectious:

  • Infectious is first recorded around 1535–45.
  • Infectious, infection, and infect all go back to the Latin infectus, past participle of the verb inficere, “to dip, dye, tint; infect, stain, poison, corrupt.”
  • The root of the Latin verb inficere is facere, “to do, make,” and is a source of a great number of English words, from fact to affection.

How do infectious diseases spread?

Infectious diseases can be spread in a variety of ways. Some infectious diseases spread in more than one way. Major ways include:

  • direct contact with an infected person, animal, or their discharges
  • direct contact with a contaminated object
  • contaminated food and water
  • disease-carrying insects

There are many types of discharges. They include secretions, such as saliva, and body fluids, such blood, urine, and semen.

Another type of discharge is respiratory droplets. These are expelled when a person coughs and sneezes, and often when they talk, laugh, or just breathe. These droplets are sometimes referred to as aerosols.

What does airborne mean?

Some of these general ways of spreading have earned special names that feature the word borne, a past participle form of the verb bear (“carry, transmit”).

A blood-borne pathogen, such as HIV and some types of hepatitis, is transmitted by the blood or other body fluids. A food-borne illness, such as salmonella, is transmitted by food. An airborne disease, like tuberculosis, is spread through the air; airborne is especially used to describe diseases spread by germs that can survive in the air for hours.

Sometimes waterborne is used to refer to diseases like cholera, which can spread through contaminated water.

Malaria (transmitted by mosquitos) is an insect-borne disease, and Lyme disease is tick-borne.

Sexually transmitted diseases (STDs)—also called venereal diseases—are transmitted through sexual contact. Blood and other body fluids are often the vehicle through which STDs spread.

A note on the word vector, which often comes up in the context of infectious diseases. A vector can refer to an insect or other organism that transmits a pathogenic fungus, virus, bacterium, and so on. In the case of malaria, the mosquito is the vector.

So, what is a contagious disease?

Recap time. Communicable diseases are infectious diseases. An infectious disease is contagious when it spreads through direct, bodily contact with an infected person, their discharges, or an object or surface they’ve contaminated.

Some key points: this transmission is typically very easy and results from close, casual contact.

Malaria is an infectious disease caused by a parasite, but it is not contagious because you don’t get it just by being around or coming into contact with an infected person. It’s usually transmitted by a mosquito bite, which transfers the parasite from the mosquito into your blood.

Tetanus is an infectious disease, but it’s not contagious because you don’t get it by shaking hands with someone who has the infection. Like malaria, it has to enter your bloodstream, such as by getting cut by a rusty nail, where the tetanus-causing bacteria can breed.

Food poisoning is an infectious, if short-lived (acute, not chronic), disease. It’s not discussed as contagious because it is spread by eating contaminated food, not by sitting across from someone who has food poisoning.

Facts about the word contagious:

  • Contagious and its related noun, contagion, are first recorded around 1350–1400.
  • Both go back to the Latin contāgiō, “touching, contact; infection.”
  • The origin of the word contact is also related to contāgiō.
  • A contagious disease is spread by contact.

Is the coronavirus contagious?

Let’s apply what we’ve learned.

  1. We know COVID-19 is a disease caused by an infection from a virus, a new type of coronavirus.
  2. This infection very easily spreads by direct and casual contact with a person, droplets, and infected surfaces.
  3. So, we can say COVID-19 is indeed a contagious disease.

That’s why we are staying at home and social-distancing the best we can! It’s also why washing your hands is so important—not only for COVID-19, but for other contagious and infectious diseases, as well as for your general health and hygiene.

Why is the word contagious important?

In everyday settings, the distinction between contagious and infectious often breaks down. Oftentimes, that’s just fine: contagious and infectious, very generally, can refer to a disease that tends to spread from one person to another.

We use the terms figuratively, too. Contagious laughter might spread across a classroom after the class clown cracks a perfectly timed joke. Or, low morale might be said to be contagious in an office after layoffs have been announced. If someone has an infectious smile, it’s simply irresistible. Infectious, in its metaphorical sense, tends to have a positive connotation. Contagious can be positive or negative, though it might skew toward the latter.

But, if contagious and infectious are sometimes used interchangeably, why do we need to make a distinction between them at all?

In some instances, especially in public health emergencies, it’s important we call out that a disease isn’t just infectious, but that it is contagious. Calling a disease contagious highlights the fact that it is very easily spread by being around people and public places—in our very normal life circumstances. This can help influence and guide behaviors to help prevent ourselves and others from getting infected, whether it’s getting a yearly flu shot or practicing social distancing.

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