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Smallpox Virus: Structure and Function

Instructor: Bridgett Payseur

Bridgett has a PhD in microbiology and immunology and teaches college biology.

Smallpox is caused by a large virus that can go through several different stages to infect cells. This lesson will discuss the different forms of the virus and how they continue the smallpox infection from cell to cell.

What Is Smallpox?

Smallpox is an infection caused by the Variola virus. It is an incredibly serious disease with a fatality rate near 30%. Early symptoms are vague, including fever, headache, and vomiting. A rash then develops and grows in intensity. A patient is contagious until the rash has completely gone away and all the scabs have fallen off.

Smallpox was the first disease to have an official vaccine to protect people from getting infected. Thanks to this vaccine, smallpox has been considered eradicated worldwide since 1977, meaning it is no longer found in nature to infect humans. Smallpox virus is only found in laboratory samples and only used under extremely strict safety measures.

Smallpox Virus Structure

Like other pox viruses, the smallpox virus is very large and has a complex life cycle. It is a brick-shaped virus with an envelope (barrier layer) that it steals from a host cell. The core of the virus is often described as dumbbell-shaped, meaning the middle of the core is narrower than the ends. The core contains the viral double-stranded DNA and proteins needed to help uncoat the virus core and replicate the virus.

Electron micrograph showing dumbbell shape of virus core.
Smallpox virus

The specific structure of the smallpox virus will vary based on whether it is inside the cell, outside the cell, or attached to the cell's surface. As the virus goes through its life cycle, it will go through several possible stages, each with different aspects, which will be discussed below.

Smallpox Infection and Replication

One of the infectious forms of the smallpox virus that invades cells is called the intracellular mature virus (IMV). As the name implies, this is a mature form of the virus that is inside the host cell. IMVs enter new host cells by fusing with the cell membrane.

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