Virulence facts for kids

Virulence is how much a pathogen (a germ that causes disease) or microorganism (a tiny living thing) can harm a host (the living thing it infects). It's like how strong a germ is at making someone sick.

In animals, virulence usually means how much damage a microbe causes to its host. The ability of a germ to cause disease is called its pathogenicity. This ability is decided by special features of the germ called virulence factors. In plants, sometimes virulence means a pathogen's ability to infect a plant that is usually resistant to diseases.

The word virulence comes from the word virulent, which means how severe a disease is. Virulent itself comes from the Latin word virulentus, meaning "a poisoned wound" or "full of poison."

From an ecological point of view, virulence is how much a parasite weakens its host. Think of it as how much energy or health the host loses because of the parasite.

How Germs Cause Harm

When we talk about germs, especially bacteria and viruses, their ability to cause disease depends on a few things:

• How many germs infect the body.
• How they get into the body.
• How the body's defenses fight them.
• Special features of the germs themselves, called virulence factors.

Many virulence factors are special proteins that germs inject into host cells. This is often done using tiny, specialized tools, like a "type three secretion system." Sometimes, the body's own strong response to an infection can also cause damage to its tissues. This is like when the body fights so hard it hurts itself, for example, during a "cytokine storm."

The virulence factors of bacteria are usually proteins or other molecules made by enzymes. These proteins are coded by genes found in the bacteria's main DNA (chromosomal DNA), in special virus DNA called bacteriophage DNA, or in small loops of DNA called plasmids. Some bacteria can even share these genes with each other using "mobile genetic elements" and "horizontal gene transfer." Scientists are looking for ways to fight infections by targeting these special virulence factors. Bacteria also use a communication system called quorum sensing to release these molecules at the same time.

How Bacteria Make You Sick

Bacteria use different methods to cause disease:

Sticking to Cells (Adhesion)

Many bacteria first need to stick to the surface of host cells. Scientists have found many bacterial and host molecules that help bacteria stick to host cells. Often, the host cell parts that bacteria stick to are important for other normal functions of the body. Because some host cells are covered in mucus or have anti-germ substances, it can be hard for some pathogens to stick directly.

Setting Up Home (Colonization)

Some strong bacteria make special proteins that let them settle and grow in parts of the host body. For example, Helicobacter pylori can live in the very acidic human stomach. It does this by making an enzyme called urease. When this bacterium settles in the stomach lining, it can lead to stomach ulcers and even cancer. The more urease Helicobacter pylori makes, the more harmful it tends to be.

Breaking In (Invasion)

Some strong bacteria make proteins that either break open host cell membranes or trick host cells into pulling the bacteria inside. This process is called endocytosis or macropinocytosis. These virulence factors help bacteria enter host cells and get deeper into the body by crossing the protective layers of tissue on the body's surface.

Stopping Immune Defenses

Many bacteria produce virulence factors that stop the host's immune system from defending itself. For example, a common trick for bacteria is to make proteins that grab onto the host's antibodies, which are part of the immune system. The polysaccharide capsule of Streptococcus pneumoniae stops immune cells from "eating" (a process called phagocytosis) the bacterium.

Making Poisons (Toxins)

Many virulence factors are proteins made by bacteria that act like poisons to host cells and cause tissue damage. For example, many food poisoning toxins are made by bacteria that can contaminate human food. Some of these poisons can stay in "spoiled" food even after it's cooked and still make people sick when eaten. Other bacterial toxins are destroyed by the heat of cooking.

How Viruses Cause Harm

Virus virulence factors help them make copies of themselves, trick the host's defenses, and spread inside the host. They can also be harmful to the host.

These factors decide if an infection will happen and how severe the viral disease symptoms will be. Viruses often need specific proteins on host cells to which they can attach. Usually, these host cell proteins then pull the virus inside the cell. Strong viruses like HIV, which causes AIDS, have ways to avoid the host's defenses. HIV infects T-helper cells, which are important immune cells. This weakens the host's immune system, leading to a state where the body can't fight off other infections. Death often happens from other infections that take advantage of the weakened immune system caused by AIDS. Some viral virulence factors allow viruses to make copies even when the host's body is trying to fight them, like during a fever. Many viruses can live inside a host for a long time without causing much harm. However, very strong strains can eventually develop through changes (mutation) and natural selection within the virus population inside a host. The term "neurovirulent" is used for viruses like rabies and herpes simplex that can get into the nervous system and cause disease there.

Well-studied examples of strong viruses include virus T4 and other T-even bacteriophages that infect Escherichia coli and other related bacteria.

The "lytic life cycle" of strong bacteriophages is different from the "temperate life cycle" of milder bacteriophages.

See also

In Spanish: Virulencia para niños