Where They Are Found

Viruses are found on or in just about every material and environment on Earth from soil to water to air. They're basically found anywhere there are cells to infect. Viruses have evolved to infect every form of life, from animal to plant and from fungi to bacteria.

However, viruses tend to be somewhat picky about what type of cells they infect. Plant viruses are not equipped to infect animal cells, for example, though a certain plant virus could infect a number of related plants. Sometimes, a virus may infect one creature and do no harm, but cause havoc when it gets into a different but closely enough related creature.

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What They Look Like

Viruses are the simplest and tiniest of microbes; they can be as much as 10,000 times smaller than bacteria. Viruses consist of a small collection of genetic material (DNA or RNA) encased in a protective protein coat called a capsid. (Retroviruses are among the infectious particles that use RNA as their hereditary material. Probably the most famous retrovirus is human immunodeficiency virus, the cause of AIDS.) In some viruses, the capsid is covered by a viral envelope made of proteins, lipids and carbohydrates. The envelopes may be studded by spikes made of carbohydrates and proteins that help the virus particles attach to host cells. Outside of a host, viruses are inert, just mere microbial particles drifting aimlessly.

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What They Are

A virus is basically a tiny bundle of genetic material—either DNA or RNA—carried in a shell called the viral coat, or capsid, which is made up of bits of protein called capsomeres. Some viruses have an additional layer around this coat called an envelope. That's basically all there is to viruses.

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Viruses of Note

• Adenoviruses are used in experimental gene therapy treatments to deliver therapeutic genes.

• Bacteriophages are being explored as tools to treat bacterial infections by targeting and destroying infectious bacteria.

• Human Immunodeficiency Virus (HIV) is responsible for the AIDS pandemic.

• Human papillomavirus causes cervical cancer.

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Single-minded Mission

Viruses exist for one purpose only: to reproduce. To do that, they have to take over the reproductive machinery of suitable host cells.

Upon landing on an appropriate host cell, a virus gets its genetic material inside the cell either by tricking the host cell to pull it inside, like it would a nutrient molecule, or by fusing its viral coat with the host cell wall or membrane and releasing its genes inside. Some viruses inject their genes into the host cell, leaving their empty viral coats sitting outside.

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Other Virus Like Things

Viruses may be referred to often as the smallest infectious things. But there are some smaller contenders. Some of the agents of plant disease lack even a viral coat and are merely small strings of plain, or "naked," RNA. These particles are called viroids. They are believed to be a more primitive version of ordinary viruses.

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DNA Disruptor

Viruses can act as miniature couriers. When they infect, they may inadvertently take up a bit of their host’s DNA and have it copied into their progeny. When the offspring viruses move on to infect new cells, they may insert this bit of accidentally pilfered DNA into the new hosts’ genome. This process is called transduction.

This can sometimes create a happy outcome. For example, the soil-dwelling bacterium Bacillus subtilis has viral genes that help protect it from heavy metals and other harmful substances in the soil.

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Contact

When viruses come into contact with host cells, they trigger the cells to engulf them, or fuse themselves to the cell membrane so they can release their DNA into the cell.

Once inside a host cell, viruses take over its machinery to reproduce. Viruses override the host cell’s normal functioning with their own set of instructions that shut down production of host proteins and direct the cell to produce viral proteins to make new virus particles.

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