Viruses

A virus is a small infectious agent that can replicate only inside the living cells of an organism. Viruses can infect all types of life forms, from animals and plants to microorganisms, including bacteria and archaea.

Most viruses are too small to be seen with a light microscope. However, electron microscopes can reveal the intricate details of a virus's structure. The term "virus" is derived from the Latin word "virus", meaning "poison".

Viruses are classified into four major groups, based on their structure. These include:

1. RNA viruses
2. DNA viruses
3. Retroviruses
4. Hepadnaviruses

RNA viruses are the most common type of virus. They are typically icosahedral or spherical in shape and range in size from 20 to 30 nanometers. RNA viruses can be divided into three subgroups:

1. Positive-sense RNA viruses
2. Negative-sense RNA viruses
3. Double-stranded RNA viruses

Positive-sense RNA viruses are the largest subgroup of RNA viruses. They are so named because their RNA is in the same form as messenger RNA (mRNA), which is used by the cell to produce proteins.

Negative-sense RNA viruses are the second largest subgroup of RNA viruses. They are so named because their RNA is in the form of complementary RNA (cRNA), which is the opposite of mRNA.

Double-stranded RNA viruses are the smallest subgroup of RNA viruses. They get their name from the fact that their RNA is double-stranded.

DNA viruses are viruses that have DNA as their genetic material. They can be either double-stranded or single-stranded. DNA viruses are typically icosahedral or spherical in shape and range in size from 20 to 30 nanometers.

Retroviruses are a type of RNA virus that can insert a copy of their RNA into the DNA of a host cell. This allows them to replicate even in the absence of a virus particle. Retroviruses are typically icosahedral or spherical in shape and range in size from 20 to 30 nanometers.

Hepadnaviruses are a type of DNA virus that can infect both animals and humans. Hepadnaviruses are typically icosahedral or spherical in shape and range in size from 20 to 30 nanometers.