What is a virus? Learn about all deadly viruses. - Seeker's Thoughts

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What is a virus? Learn about all deadly viruses.

The 2019 novel coronavirus (2019-nCoV) was behind disastrous outbreak- which the world health Organization had declared a pandemic, was named after the family of viruses it belongs to.

The term “coronavirus” may have initially been unfamiliar to many, but most everyone has encountered milder forms of such viruses, of which four strains cause about a fifth of common cold cases.

Other types of cause diseases that are endemic in certain animal populations. But until less than two decades ago, all known human verities caused illness so mile that coronavirus research was something of a backwater.

In these years since several viruses like SARS, MERS first emerged as a severe global health threat, research has been studying their molecular biology in an effort to answer such questions.

What are viruses?

Viruses are very small – 100 times smaller than the average bacterium, so small that they can’t be seen with an ordinary microscope. 

Viruses can only exert influence by invading a cell because they’re not cellular structures. They lack the ability to replicate on their own. So viruses are a merely tiny packet of DNA or RNA genes enfolded in a protein coating, on the hunt for a cell they can dominate.

Who they can infect?

Viruses can infect every living thing – from plants and animals down to the smallest bacterium. For this reason, they always have the potential to be dangerous to human life.

Still, they don’t become truly treacherous until they infect a cell within the body. This infection can happen in several ways: by air, via carrier insects like mosquitoes or birds, or by the transmission of body fluids such as saliva, blood or semen.

Once a virus infects a cell, it tries to take over its host completely, a virus lodged in a cell replicates and reproduces as much as possible; with each new replication, and the host cell produces more viral material than it does normal genetic material. Left unchecked, the virus will cause the death of the host cell. Viruses will also spread to nearby cells and begin the process again.

The human body does have some natural defenses against a virus. A cell can initiate RNA interference when it detects viral infection, which works by decreasing the influence of the virus’s genetic material in relation to the cell’s usual material.

The immune system also kicks into gear when it identifies a virus by producing antibodies that bind to the virus and render it unable to replicate. The immune system also releases T-cells, which work to kill the virus antibiotics have no effect on viruses, though vaccinations will provide immunity.

      Viruses like COVID -19 invade on Weak Immune system

Viruses can evolve quickly?

Unfortunately for humans, some viral infections outpace the immune system. Viruses can evolve much more quickly than the immune system can, which give them a leg up in uninterrupted reproduction. And some viruses, such as HIV, work essentially by tricking the immune system. Viruses cause many diseases, including colds, measles, chickenpox, HPV, herpes, rabies, SARS and the flu. Though they’re small, they pack a big punch – and they can only sometimes be sent into exile.

Origin of viruses

Viruses co-exist with life wherever it occurs. They have probably existed since living cells first evolved. The origin of viruses remains unclear because they do not form fossils, so molecular techniques have been the most useful means of hypothesizing how they arose. However, these techniques rely on the availability of ancient viral DNA or RNA but most of the viruses that have been preserved and stored in laboratories are less than 90 years old.

Molecular methods have only been successful tracing the ancestry of viruses that evolved in the 2oth century. New groups of viruses might have repeatedly emerged at all staged of the evolution of life.

Three main theories speculate on the origin of viruses

Regressive theory 

Viruses may have once been small cells that parasitized larger cells. Over time, genes not required by their parasitism were lost. The bacteria rickettsia and chlamydia are living cells that, like viruses, can reproduce only inside host cells. They lend credence to this theory, as their dependence on parasitism is likely to have caused the loss of genes that enabled them to survive outside a cell. 

Cellular origin theory 

Some viruses may have evolved from bits of DNA or RNA that "escaped" from the genes of a larger organism. The escaped DNA could have come from plasmids—pieces of DNA that can move between cells—while others may have evolved from bacteria. 

Coevolution theory 

Viruses may have evolved from complex molecules of protein and DNA at the same time as cells first appeared on earth and would have depended on cellular life for many millions of years.


A virus particle, also known as a virion, consists of genes made from DNA or RNA which are surrounded by a protective coat of protein called a capsid. The capsid is made of many smaller, identical protein molecules which are called capsomers. The arrangement of the capsomers can either be icosahedral (20-sided), helical or more complex. There is an inner shell around the DNA or RNA called the nucleocapsid, which is formed by proteins. Some viruses are surrounded by a bubble of lipid (fat) called an envelope.


Viruses are among the smallest infectious agents, and most of them can only be seen by electron microscopy. Most viruses cannot be seen by light microscopy (in other words, they are sub-microscopic); their sizes range from 20 to 300 nm. They are so small that it would take 30,000 to 750,000 of them, side by side, to stretch to one cm.

 By contrast, bacterial sizes are typically around 1 micrometer (1000 nm) in diameter, and the cells of higher organisms a few tens of micrometers. Some viruses such as mega viruses and pandoraviruses are relatively large. At around 1 micrometer, these viruses, which infect amoebae, were discovered in 2003 and 2013. They are around a thousand times larger than influenza viruses and the discovery of these "giant" viruses astonished scientists.

Some deadly viruses on Earth

Marburg virus

Scientists identified Marburg virus in 1967 when small outbreaks occurred among lab workers in Germany who were exposed to infected monkeys imported from Uganda. Marburg virus is similar to Ebola in that both can cause hemorrhagic fever, meaning that infected people develop high fevers and bleeding throughout the body that can lead to shock, organ failure and death.

The mortality rate in the first outbreak was 25%, but it was more than 80% in the 1998-2000 outbreak in the Democratic Republic of Congo, as well as in the 2005 outbreak in Angola, according to the World Health Organization (WHO). 

The first known Ebola outbreaks in humans struck simultaneously in the Republic of Sudan and the Democratic Republic of Congo in 1976. Ebola is spread through contact with blood or other body fluids, or tissue from infected people or animals.

 The known strains vary dramatically in their deadliness, Elke Muhlberger, an Ebola virus expert and associate professor of microbiology at Boston University, told Live Science.

One strain, Ebola Reston, doesn't even make people sick. But for the Bundibugyo strain, the fatality rate is up to 50%, and it is up to 71% for the Sudan strain, according to WHO.

The outbreak underway in West Africa began in early 2014, and is the largest and most complex outbreak of the disease to date, according to WHO.


Hantavirus pulmonary syndrome (HPS) first gained wide attention in the U.S. in 1993, when a healthy, young Navajo man and his fiancée living in the Four Corners area of the United States died within days of developing shortness of breath. A few months later, health authorities isolated Hantavirus from a deer mouse living in the home of one of the infected people. More than 600 people in the U.S. have now contracted HPS, and 36% have died from the disease, according to the Centers for Disease Control and Prevention.

The virus is not transmitted from one person to another, rather, people contract the disease from exposure to the droppings of infected mice.

Previously, a different Hantavirus caused an outbreak in the early 1950s, during the Korean War, according to a 2010 paper in the journal Clinical Microbiology Reviews. More than 3,000 troops became infected, and about 12% of them died.

While the virus was new to Western medicine when it was discovered in the U.S., researchers realized later that Navajo medical traditions describe a similar illness, and linked the disease to mice

Two vaccines are now available to protect children from rotavirus, the leading cause of severe diarrheal illness among babies and young children. The virus can spread rapidly, through what researchers call the fecal-oral route (meaning that small particles of feces end up being consumed).

Although children in the developed world rarely die from rotavirus infection, the disease is a killer in the developing world, where rehydration treatments are not widely available.

The WHO estimates that worldwide, 453,000 children younger than age 5 died from rotavirus infection in 2008. But countries that have introduced the vaccine have reported sharp declines in rotavirus hospitalizations and deaths.

         Viruses are secretly controlling the earth

The virus that causes the severe acute respiratory syndrome, or SARS, first appeared in 2002 in the Guangdong province of southern China, according to the WHO. The virus likely emerged in bats, initially, then hopped into nocturnal mammals called civets before finally infecting humans. After triggering an outbreak in China, SARS spread to 26 countries around the world, infecting more than 8000 people and killing more than 770 over the course of two years. 

The disease causes fever, chills and body aches, and often progresses to pneumonia, a severe condition in which the lungs become inflamed and fill with pus. SARS has an estimated mortality rate of 9.6%, and as of yet, has no approved treatment or vaccine. However, no new cases of SARS have been reported since the early 2000s, according to the CDC


 SARS-CoV-2 belongs to the same large family of viruses as SARS-CoV, known as coronaviruses, and was first identified in December 2019 in the Chinese city of Wuhan. The virus likely originated in bats, like SARS-CoV, and passed through an intermediate animal before infecting people. 

Since its appearance, the virus has infected tens of thousands of people in China and thousands of others worldwide. The ongoing outbreak prompted an extensive quarantine of Wuhan and nearby cities, restrictions on travel to and from affected countries and a worldwide effort to develop diagnostics, treatments, and vaccines.

The disease caused by SARS-CoV-2 called COVID-19 and has an estimated mortality rate of about 2.3%. People who are older or have underlying health conditions seem to be most at risk of having severe disease or complications. Common symptoms include fever, dry cough and shortness of breath, and the disease can progress to pneumonia in severe cases.


The virus that causes Middle East respiratory syndrome, or MERS, sparked an outbreak in Saudi Arabia in 2012 and another in South Korea in 2015. The MERS virus belongs to the same family of viruses as SARS-CoV and SARS-CoV-2 and likely originated in bats, as well. The disease infected camels before passing into humans and triggers fever, coughing and shortness of breath in infected people. 

MERS often progresses to severe pneumonia and has an estimated mortality rate between 30% and 40%, making it the most lethal of the known coronaviruses that jumped from animals to people. As with SARS-CoV and SARS-CoV-2, MERS has no approved treatments or vaccines. 

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