Establishment of Disease: Entry, Dose & Virulence

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  • 0:05 Disease Due to an Infection
  • 0:27 Pathogenicity
  • 2:03 Virulence, Lethal…
  • 4:08 Entry into a Host
  • 5:26 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov
This lesson will delve into what predisposes an individual to get sick. We'll talk about things like the entry, dose, and virulence of a pathogen and how they play a role in the establishment of a disease.

Disease Due to an Infection

I hope that you are over 21, because we will have to enter a casino to get some of the points in this lesson across. If you're not, then maybe you can borrow a fake ID or don a fake beard to get in. This lesson will be discussing how it is that you may get a disease due to a few key factors involved in the process of infection.

Pathogenicity

To clarify, an infection is the first step in the establishment of disease. If you don't get infected by a pathogenic organism, then you won't get sick unless, of course, you get sick due to a non-infectious disease process, such as diabetes. However, this lesson will focus in on how infectious causes establish a disease at the very onset of the entire process.

There are several things that influence how well-suited a microbe is to causing a disease. The ability of a microbe to cause disease is termed pathogenicity. This is a qualitative description. For example, some microbes are pathogenic, meaning they can harm you and cause you a disease. They're like the alcoholic drinks at the casino that will end up hurting you the morning after you drink them. This is in contrast to non-pathogenic microbes that will not cause you any harm. This is akin to the non-alcoholic drinks that you may encounter at a casino.

Again, pathogenicity is a qualitative term and a relative one as well. For example, in a normal healthy person, certain microbes may be considered to be non-pathogenic - that is to say they are unable to cause a disease in that person. But, if a person is immunocompromised due to some other infection, notably HIV, even what was at one point considered to be a non-pathogenic microbe can now cause a disease in a person that is immunocompromised.

Virulence, Lethal Dose, and Infectious Dose

While pathogenicity is a qualitative term, there is a more quantitative term that is sometimes used. This term is known as virulence, and it is the degree of pathogenicity of a microbe. It's essentially a measurement of a microbe's disease-producing potential. You can liken virulence to the odds of winning a certain game at the casino. The more virulent an organism is, the higher the chances are that you'll get a nasty disease due to that specific microbe.

Virulence is sometimes measured quantitatively by a term known as LD50 or lethal dose, 50%. This is the number of microbes necessary to kill 50% of a population infected with that microbe. The lower the LD50, the more virulent something is. That's because we need a smaller dose, or a smaller number of microbes, to kill a set number of individuals.

For instance, you know that cyanide is really poisonous or virulent, so to speak, when compared to something like sugar. Therefore, if you were to give a test population of 1,000 people either sugar or cyanide to try and kill 50% of them, you would need them to eat far more sugar than cyanide to kill 50% of the sample population. Therefore, the LD50 of cyanide is far lower than the LD50 of sugar.

Virulence can also be measured by something known as ID50 or infectious dose, 50%. This is the number of microbes necessary to infect a host in 50% of a sample population. Again, the lower the infectious dose, the more virulent a pathogen is since we need fewer amounts of that pathogen to cause an infection in the host.

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