Vaccsbook

Viruses

Viruses are sub-microscopic infectious particles which can cause disease of all levels of severity. A single viral particle is called a Virion.

Viruses are not considered to be microbes or organsims. While they they can exist in the environment, they cannot multiply by themselves; viruses must invade the "host" cells of other living organisms, most often the cells lining the the nose, mouth, eye, lungs, stomach and bowel. Viruses can also be carried in bodily fluids and transmitted to others by contact with those fluids.

Viral structure is much simpler than that of microbes, consisting of just genetic material, DNA or RNA , inside a shell made of proteins.

Viruses enter host cells by using antigens ("keys") on their shell which match specific receptors ("locks") on the host cells. The well-known Spike of the SARS Cov2 Coronavirus is such an antigen.

Once inside, they multiply rapidly using the host cell's metabolic processes, primarily the translation of mRNA into proteins.

New virions are released from host cells either:
a) by squeezing out of the host cell during which they acquire an "envelope" covering their shell, or
b) by completely disrupting the host cell and causing it to disintegrate thereby releasing "naked" virions (without an envelope).

Viral structure is much simpler than the cells of microbes, consisting of just genetic material - DNA or RNA - inside a shell made of proteins.
Viruses are commonly typed according to:

1. whether their genetic material is have DNA or RNA, (never both within the same virus-type)
2. whether the DNA/RNA occurs as a single strand or as double strands
3. how the DNA/RNA strand is folded or coiled
4. the shape of the protein capsule.
5. whether the shell is enveloped in a membrane or not.

RNA viruses
      have their RNA processed directly by a host cell to create more virions.
An exception is the Retrovirus type (e.g. HIV) whose RNA is first transcribed into DNA, which then enters the nucleus of the host cell where it is combined with host DNA, and subsequently transcribed back into mRNA.

DNA viruses
      must first be transcribed into mRNA, which is then processed by the host cell to create more virions.

Different virus types vary in size from 20 to 200 nanometers (nm) . A nm is 1 billionth of a meter, or 1 millionth of a millimeter, a human hair is 100,000 nanmeters thick, so at least 500 times the largest virion. The best light microscope can only see as small as 500 nanometers. Some virus types have size range, e.g. the SARS-Cov2 virus varies from 60nm to 140nm.

Examples of viruses which have largely been controlled in developed countries with vaccines are: Measles, Mumps, Polio, Chicken Pox, Shingles, Influenza, Hepatitis, and Rabies.
Other viruses for which (as of 2022) there is no widely available vaccine include HIV/AIDS, West Nile, Zika, most Coronavirus, and a variety of unnamed viruses which cause self-limiting respiratory or intestinal symptoms.
SARS-Cov2, the COVID-19 virus, is a Coronavirus which has mutated into several variants most of which are susceptible to some degree to newly developed and/or exprimental vaccines.


In the late 1800s, scientists realized that microbes much smaller than bacteria were causing infections, and described them as a "virus", from the latin for "poison". However, viruses were too small to be seen with an ordinary microscope.

During the Spanish Flu pandemic of 1918/19, no one knew the cause.
It wasn't until the invention of the electron microscope in early 1930s, that viruses could actually be visualized. Only then was the 1918/19 pandemic thought "likely" to have been caused by the Influenza virus. There have been (much less severe) influenza pandemics in 1957, 1968, 1977, and 2009, during which the virus was confirmed as Influenza.
The nature of the 2019+ Coronavirus pandemic is raising some questions about which virus really caused the 1918/19 pandemic.