Bridgett has a PhD in microbiology and immunology and teaches college biology.
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.
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.
Some intracellular viruses have a double-envelope. These are termed intracellular enveloped viruses (IEVs). IEVs can attach to the outside of the host cell, at which point they are no longer intracellular and are called cell-associated enveloped viruses (CEV). If the IEV completely leaves the cell, it is called an external enveloped virus (EEV). The EEV is also an infectious form of the smallpox virus. It is taken into the cell by a process called endocytosis. Endo means into, and cyto refers to cell. Endocytosis is, therefore, a process to help get things into a cell. It is often described as the cell eating something.
Once inside a host cell, the smallpox virus will release its DNA from the core with help from both cellular and viral proteins. The DNA is copied and used to make new viral proteins. The virus then forms into one of the different structures described above and moves on to infect a new cell. Most of the virus particles formed will be in the IMV structure.
One hypothesis explaining the various structures and methods of infection of the smallpox virus is that it provides a greater range of cells to be infected. Each of the different methods of infection seem to use a different receptor, giving the virus more options when looking for new cells to infect.
Smallpox is a very dangerous infection that has, thankfully, been eradicated. Like other members of the pox virus family, it is very large and has a variety of different possible forms. The different forms infect new cells in distinct ways, potentially giving smallpox the ability to infect a variety of types of cells.
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