Dear Reader:

Viruses are by far the most abundant biological entities on Earth. They infect fungi, bacteria, plants and animals. Viruses that infect plants are harmless to animals, and most viruses that infect other animals are harmless to humans.

Some viruses infect one species only (being species-specific, having a narrow host range), for example, the smallpox virus infects only humans. Other viruses infect a few or many species (having a broad host range), for example, the rabies virus infects bats, coyotes, horses, monkeys, rabbits, skunks, racoons, foxes, wolves, cattle, cats, dogs and humans.

Zoonotic viruses are viruses that originated in other animals and transitioned from that other animal to humans through insect  bites, animal bites, contact with feces and bodily fluids (mucus, saliva, blood, urine, semen), and inhaling (respiratory transmission).

Viral populations don’t grow through cell divisions because they are not cells. They grow by using the host cell’s mechanism to multiply. First a virus attaches itself to the host by its surface protein binding to a specific receptor on the outside of the host’s cell. The virus then fuses with the host’s  cellular membrane (or gets taken in by the host’s cell), and the virus finds itself inside the host’s cell. A virus is made of genetic material (DNA or RNA) surrounded by a protein shell called capsid. As the virus enters the host cell, it loses its capsid and all it is by that point is genetic material. Next, the viral genetic material uses the host cell’s resources to synthesize more genetic material and proteins, replicating itself this way, multiplying and finally they getting itself out of the host cell.

DNA viruses contain DNA as their genetic material while RNA viruses contain RNA as their genetic material. DNA viruses are larger, most contain a double stranded DNA, and they replicate themselves in the nuclei of the host’s cells. RNA viruses are smaller, most contain a single stranded RNA, and they replicate themselves in the cytoplasm of the host’s cells.

Some viruses cause no apparent changes in the host’s cells, being inactive there. Some awaken from their dormant state from time to time and cause distressing changes to the host’s cells, for example, the herpes viruses.

Six species of human coronaviruses have been identified, with one of the species subdivided into two different strains (a strain is a genetic variant, a subtype),  making 7 strains of coronaviruses altogether. 4/7 are mild forms that continually circulate in human population all over the world, producing the symptoms of the common cold. 3/7 produce symptoms that are potentially severe:

1. Middle East Respiratory Syndrome-related Coronavirus (MERS-CoV)- almost 2,500 cases, 34.5% mortality rate.

2. Severe Acute Respiratory Syndrome Coronavirus (SARS-CoV)- more than 8,000 cases, about 10% mortality rate.

3. Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2)- almost 4 millions today and almost 300,000 dead (today,  May 8, 2020).

The three listed coronaviruses above, all originated in bats (a zoonotic origin). It is not known if SARS-Cov-2, the virus behind Covid-19, transferred from bats to humans directly, or through an intermediate animal/ host.

All coronaviruses are RNA viruses with a broad range host range, infecting birds, mammals (bats, cows, pigs, ferrets, rabbits, laboratory mice and rats, and humans), but there is no significant evidence of the current Covid-19 coronavirus (SARS-Cov-2) infecting farm animals and pets so far.

SARS-Cov-2 (Cov-2 for short), infect human lungs by its surface binding a “spike” on its surface protein with a human enzyme (ACE2) which is abundant in human lungs cells. The higher the density of ACE2 in a group of cells (tissue), the more Cov-2 bind to the tissues and the more severe the symptoms suffered by the human host.

Most of those who die of Covid-19 have pre-existing conditions, including hypertension (high blood pressure), diabetes, and cardiovascular disease (diseases of the heart and blood vessels), chronic respiratory diseases (such as COPD where patients suffer from chronic poor air flow as a result of a breakdown of lung tissue).

In people younger than 50, the risk of death is less than 0.5%. In people older than 70, the risk is 16 times greater, 8%. According to a data table called “case fatality rates (%) by age and country or US state” in Wikipedia, in children 0-9, the case fatality rate is zero in most countries, 0.1% in Italy, and 0.9% in Sweden. In children and adults 10-19 years old, the case fatality rate is zero in most countries, 0.2% in China and the Netherlands, and 0.4% in Spain. In adults 20-29, the case fatality rate is zero in most countries, 0.1% in Italy and the Netherlands, 0.2% in China, Spain and the US, and 0.4% in Sweden.

More Later.

anita