The Difference Between Communicable and Infectious Diseases

It's More Than Just Semantics.

Virus, Computer Artwork
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While the words "infectious" and "contagious" are often used interchangeably to describe illnesses, they actually mean two very different things.

Infectious Disease

Put simply, an infectious disease is a disease that can cause an infection. That is, when a microorganism comes into your body, and makes itself comfortable. For bacteria or fungi, this means dividing and growing new cells at an exponential rate.

Viruses, on the other hand, have an added hurdle of entering human cells and taking over their control centers so that they can make more of themselves. Regardless of the agent or mechanism, the affect is internal: pathogens come inside your body and spread throughout. Sometimes this can lead to symptoms. Sometimes it doesn’t.

Invisible Infections 

The human papillomavirus is an example of a pathogen that can cause an infection but not necessarily symptoms. Nearly all adults in the United States will get infected with HPV, but most don't even realize it. While the virus can lead to genital warts or cancer in some people, the vast majority of the time it doesn't cause any symptoms at all. Your body is able to clear the infection without you ever getting sick—but you could still spread it to others.

Communicable Disease

A communicable disease is a contagious one. The effect is external. If someone catches the illness, they can get sick and spread the pathogen—be it a cold, virus, or some other disease-causing agent—onto the next person.

This can lead to small, isolated outbreaks or full-scale pandemics.

An example of this happens each and every year in the United States from roughly October to May: the flu. As influenza viruses are passed from person to person and via contaminated objects, the virus spreads far and wide. For every one person with the flu, another one to two others will likely become infected if they aren't immune.

This rate of how fast a pathogen can spread is called the basic reproductive number, or R0, and it depends on a wide variety of factors, including how the microbe travels to new people.

Methods of Transmission

Nature has no shortage of creativity in how pathogens can travel through a population. These are just a few of the most common methods microbes use to spread.

Person-to-Person

Pathogens that go from one person to another can be transmitted a number of ways, such as through respiratory droplets like coughing or sneezing, sexual activity, contact with blood, or from mother to child during pregnancy, birth, or breastfeeding.

Active illness where you're sneezing or coughing a lot can give the microbe more opportunities to spread, but you don't have to have symptoms to be contagious. You don't even have to be around. Measles, for example, can be transmitted up to four days before you even develop the telltale rash, and the virus can stay in the air for as long as two hours after you have left the room.

Vectors

Some microbes aren't spread from person-to-person, but rather along a more circuitous person-vector-person path. Responsible for millions of illnesses each year, mosquitoes are one of the world's most common vectors.

 Malaria, for example, is spread by mosquitoes who become infected after biting someone with the disease, and then they, in turn, pass the parasite onto the next person they bite.

The presence of mosquitoes alone isn't enough to spread diseases. They are merely the pass-through. If there aren't any people around with malaria, the mosquitos can't spread the disease.

Fomites

Some diseases don't even need a living thing to infect new people—just a surface to hang onto. One of the reasons hand-washing is so important to prevent disease is because there is a ton of objects we touch every day that are rife with germs that can make you sick.

During cold and flu season, absent-mindedly wiping a runny nose and then touching a doorknob is a common way for the viruses to hop a ride to the next person. When you come behind them to open the same door, the viruses get onto your skin and wait for an opportunity to get inside your body—often when you are touching your nose or rubbing your eyes.

Contaminated Food or Water

A particularly gross way germs travel is through the fecal-oral route—that is, through food or water that's contaminated by fecal matter, i.e. poop. Food or drinking water can become contaminated if infected people don't wash their hands before doing food prep or if they don't have adequate sanitation when going to the bathroom. This is a frequent reason for food poisoning.

Infectious But Not Communicable

While all communicable diseases are infectious, not all infections are communicable. Tetanus, for example, can cause an infection, but a person with tetanus can't spread it to other people. The bacteria live in dirt and dust, and get inside your body through abrasions like cuts, scraps, or punctures. While the pathogen can lead to a very serious infection and illness in individuals, it will almost certainly never cause a worldwide pandemic.

Sources: 

Mayo Clinic. Infectious Diseases

Mount Sinai Hospital. Methods of Disease Transmission. Department of Microbiology. 

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