The Differences Between Infection and Disease

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  • 0:08 Disease By Pathogen
  • 0:38 What Is an Infection
  • 1:35 What Is Disease?
  • 3:39 The Nuances of Disease
  • 4:49 Opportunistic Infections
  • 6:35 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov

Artem is a doctor of veterinary medicine and has taught science and medicine at the college level.

This lesson will go over the differences between an infection and a disease. We'll cover everything from prions and infectivity to opportunistic infections and idiopathic disease. We'll also learn what the etiology of disease is and much more.

Disease by Pathogen

If you're a fan of ancient books or of Hollywood tales, then you've undoubtedly heard of something known as the Trojan horse. This was a device used by the Greeks to outwit the Trojans during the Trojan War. In a similar fashion, we'll discover in this lesson that any pathogen, or agent of disease, that is out to get you uses a sequence of events similar to the one the Greeks devised to destroy the city of Troy - except these pathogens are out to destroy your body instead of a city.

What Is an Infection?

When a pathogen enters its host and multiplies inside of it, we term that process an infection. A pathogen can be anything you can imagine, such as a virus, bacterium, fungus, parasite, or even a prion, which is an abnormally shaped protein that causes disease.

The entire process of infection can be likened to the men inside of the Trojan horse entering the city of Troy. When the horse enters the city, soldiers jump out of it, or multiply out of it, and infect the city. The ease by which an infectious agent can enter, survive and multiply within a host is known as infectivity. In our case, because the soldiers inside the Trojan horse were carried into the city and easily survived and multiplied within it, they would be deemed 'highly infective.' However, the fun has only just begun at that point.

What Is Disease?

After the soldiers hidden away in the horse have multiplied out of it, so to speak, they run towards the city gates and open them up, thereby allowing the entire Greek army to come into the city. Once the soldiers of the Greek army run into the city, they begin to slaughter everyone and anyone that stands in their way. The people the Greeks kill are the citizens of Troy.

The citizens of the host city are like our own body's cells, tissues and organs. They are being attacked and destroyed by the pathogen. When your own body's receptors, cells, tissues or organs are damaged, destroyed or inactivated, resulting in an abnormal state of function, health and overt clinical signs and symptoms, we term that process a disease.

There's only one catch here. Disease is a two-way street. The citizens of Troy don't just lay down their arms as the Greeks attack. They fight as hard as they can for their home, their city and life. Likewise, our body has something known as an immune system, which fights off pathogens trying to freely multiply within our body. The problem is the immune system is sometimes not very picky in how it wages war.

You can liken our immune system, especially in the early stages of a disease, to the citizens of Troy showering fire arrows down upon the Greeks. While the fire arrows will certainly kill plenty of enemy soldiers, those same arrows can inadvertently strike a citizen of Troy or cause a building to catch fire. Basically, they cause destruction and mayhem on both sides of the war.

Likewise, inside of our own body, the immune system not only damages the invading pathogen but also our own body's cells, tissues and organs. Therefore, our immune system contributes to the entire process of disease to a great extent. It's definitely not a perfect system by any means, as I am sure you can now clearly tell.

The Nuances of Disease

Now, before we move on to something else, I want you to look over the definition of disease one more time. Disease is when your own body's receptors, cells, tissues or organs are damaged, destroyed or inactivated, resulting in an abnormal state of function, health and overt clinical signs and symptoms. There's something missing in that definition from the context of this lesson. Nowhere did I say that a disease must be caused by an infection.

I know for a fact you can think of at least one disease that doesn't have to be caused by a pathogen. The list is actually quite long, but here are some examples:

  • Diabetes
  • Cushing's disease
  • Heart disease
  • Cancer

And so on. Again, an infection is just one potential cause of disease but not the only one. In fact, sometimes we don't even know the root cause of a disease, or the etiology of it. Many times, a disease is idiopathic, meaning it's a disease that has no known cause.

Opportunistic Infections

Finally, before ending this lesson, like the Greeks ended the Trojan War, I'd like to cover one more important point. Earlier on, I mentioned the fact that the Greek soldiers hiding inside of the horse opened the gates to the city to let more soldiers in. I did this for a reason I'll get to in just a second. You need to realize that virtually any infection that causes a disease, and even one that doesn't cause any overt signs of a disease, may lead to something known as an opportunistic infection.

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