What is Virulence?

Instructor: Jennifer Gilley
Virulence factors contribute to a pathogen's virulence, or ability to cause disease. This lesson will describe some of the different virulence factors and will use different HIV strains to demonstrate varying levels of virulence.


We've all seen advertisements for various products that claim to kill 99% of germs. So what is a germ exactly? A germ is term used to describe small organisms, like bacteria and viruses, that can make us sick. In biological terms, a germ is also known as a pathogen. There are several different types of pathogens that cause various different illnesses, ranging from the common cold to even cancer. Pathogens affect us differently depending on their virulence. Virulence is a term used to describe how effective a particular pathogen is at making you sick. The more virulent a pathogen is, the more negatively it will affect your health.

Virulence factors are features of pathogens that determine how virulent a pathogen is. The more virulence factors a pathogen has, the more likely it is to cause disease or illness. Think of these virulence factors as specialized weapons that pathogens have. Each weapon is different but gives the pathogen an advantage against our immune system. The same is true for an army with specialized weapons, such as tanks, drones, missiles and grenade launchers. The more weapons they have, the more destructive they can be.

HIV Virulence

As mentioned above, some pathogens are more virulent than others. In fact, the same disease can be caused by different strains or versions of a pathogen. Human immunodeficiency virus (HIV), which is responsible for AIDS (acquired immune deficiency syndrome), is a good example. The HIV virus can be divided into two different groups, HIV-1 and HIV-2, based on genetic differences between the two. Both are transmitted in similar ways and can cause AIDS. However, HIV-1 is responsible for most of the HIV infections around the world. In fact, HIV-2 is less likely to lead to AIDS because it is not transmitted as easily, and it progresses much slower than HIV-1. Therefore, HIV-1 is more capable of causing AIDS because it is more virulent than HIV-2.

Virulence Factors

So why is one strain of HIV more effective at causing AIDS than the other? The difference is due to virulence factors, which contribute to a pathogen's ability to cause disease. Each virulence factor acts as a specialized weapon against our immune system's ability to fight illness. There are several different types of virulence factors, which may or may not be present on a particular pathogen.

Adhesion Factors

In order to infect a cell, a pathogen must first be able to latch onto the host cell. Adhesion factors are specialized proteins that help a pathogen grab onto a host cell. These adhesion factors act as a kind of glue that helps a pathogen attach or adhere to the cell it's trying to infect. Therefore, adhesion factors, if present, enhance a pathogen's ability to infect a host cell.

Adhesion Factors Help Pathogens Bind to a Host Cell
Action of Adhesion Factors


Another class of virulence factors are called toxins. Toxins are chemicals that are released from bacteria that can damage host tissue in a multitude of ways. For example, the bacteria associated with causing the disease botulism releases a toxin called botulinum toxin. This toxin interferes with the nervous system and can cause muscle paralysis.

Oddly enough, some people actually get this same toxin injected into their face! Why would anyone want to do that? Well, Botox (short for botulinum toxin) has been used to temporarily reduce the appearance of wrinkles by blocking muscle contraction near the site of injection. As a result, the wrinkles relax and soften. So next time you hear the term Botox, you'll know it's actually a virulence factor used for cosmetic purposes!

To unlock this lesson you must be a Member.
Create your account

Register to view this lesson

Are you a student or a teacher?

Unlock Your Education

See for yourself why 30 million people use

Become a member and start learning now.
Become a Member  Back
What teachers are saying about
Try it risk-free for 30 days

Earning College Credit

Did you know… We have over 160 college courses that prepare you to earn credit by exam that is accepted by over 1,500 colleges and universities. You can test out of the first two years of college and save thousands off your degree. Anyone can earn credit-by-exam regardless of age or education level.

To learn more, visit our Earning Credit Page

Transferring credit to the school of your choice

Not sure what college you want to attend yet? has thousands of articles about every imaginable degree, area of study and career path that can help you find the school that's right for you.

Create an account to start this course today
Try it risk-free for 30 days!
Create An Account