Virulence to the cell it’s trying to

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.

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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!

Antiphagocytic Factors

There are also other virulence factors that help pathogens evade the host’s immune system.

Some of our immune cells are responsible for finding and eating pathogens that have invaded our body. This process called phagocytosis (cellular eating) ensures that a pathogen is destroyed once it has been engulfed by our immune cells. However, some pathogens have come up with a way to outsmart our immune system.Antiphagocytic factors are structures or chemicals associated with a pathogen, which prevent phagocytosis by our immune cells. One example of an antiphagocytic factor is a capsule found on some bacteria. This capsule coats the outside of the bacteria making it very slippery. This makes it hard for our immune cells to eat and destroy the bacteria.

It’s like a farmer trying to catch a pig covered in Vaseline. Therefore, antiphagocytic factors, such as capsules, increase the virulence of a pathogen by helping them escape phagocytosis.

How Immune Cells Phagocytose Pathogens
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