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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.

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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.

Entry Into a Host

Regardless of a microbe’s pathogenicity or virulence, it must first encounter the host and then enter it in order to have a chance of establishing a disease after infection.

If you don’t enter a casino, you’ll never have any chance of winning any single game. The way by which a microbe encounters and gains access into a host is widely variable.Some microbes love to play craps at the casino.

They literally use feces-contaminated water and food in order to gain entry into your body. Other microorganisms love to play slots, meaning they love to find little slots, or openings in your skin, they can burrow into in order to enter your body. You’ve also probably heard the accusation that casinos pump higher-than-normal amounts of oxygen into their rooms in order to get people to stay. Well, some microbes take to the air to gain entry into your body so they can stay for a while.As you can tell, the methods of entry are many.

Once inside of the body, many microbes will also need to enter specific parts of the body, like organ systems or cells, in order to fully infect a person and establish a disease.

Lesson Summary

As a recap of this lesson, don’t forget that the ability of a microbe to cause disease is termed pathogenicity. This is a qualitative description. 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 its disease-producing potential.

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 is, the more virulent something is. That’s because we need a smaller dose, or a smaller number of microbes, to kill a certain number of individuals.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.

Learning Outcomes

Following this lesson, you will be able to:

  • Define pathogenicity and describe why it is a relative term
  • Explain the measurements of virulence: LD50 and ID50
  • Identify ways that a microbe can enter a host

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