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Pandemic | Informational Series | E2 | Virus



Informational Series Episode 2
Source: Science Alert

Pandemics, in epidemiological sense, are spread of an infectious disease which engulfs various regions and infects unexpectedly huge number of people. This causes a lot of destruction to humans and resources. Any disease can become a pandemic. But we do not come across a pandemic very often. This is because disease which becomes pandemic has a certain characteristics to it. Let’s explore these in today’s episode.

Let’s first understand the terms used 


In conventional epidemiological sense, anything that can produce a disease is a pathogen. It may be called a germ or an infectious agent. Generally, the term is used to describe an infectious micro-organism or agent, such as a virus, bacterium, protozoan, prion, viroid, or fungus.
Small animals, such as certain kinds of worms and insect larvae, can also produce disease. However, these animals are usually, in common parlance, referred to as parasites rather than pathogens.


An infectious disease caused by a virus, which particularly attacks respiratory system is known as Influenza. It is commonly known as "the flu". The most common symptoms include: high fever, runny nose, sore throat, muscle and joint pain, headache, coughing, and feeling tired. These symptoms typically begin two days after exposure to the virus and most last less than a week. Three of the four types of influenza viruses affect humans: Type A, Type B, and Type C. Type D has not been known to infect humans, but is believed to have the potential to do so.

The two most common causative agents of infectious disease are the virus and bacterium. Both of these pathogens are invisible to the naked eye, allowing for their stealthy transfer from person to person during an outbreak of a contagious disease. As we are witnessing the Corona Virus Pandemic, let’s turn our focus on Viral Pandemics.

How exactly does a virus functions?

Viruses can infect all types of life forms, from animals and plants to microorganisms, including bacteria and archaea. A virus replicates only inside the living cells of an organism. More than 6,000 virus species have been described in detail until now, of the millions of types of viruses in the environment. Viruses are found in almost every ecosystem on Earth and are the most numerous type of biological entity. 

When not inside an infected cell or in the process of infecting a cell, viruses exist in the form of independent particles, or virions, consisting of:

(i) The genetic material, i.e. long molecules of DNA or RNA that encode the structure of the proteins by which the virus acts;
(ii) a protein coat, the capsid, which surrounds and protects the genetic material; and in some cases
(iii) an outside envelope of lipids. 

Anatomy of Corona Virus
Corona Virus | Source: How Stuff Works

 But these virus are not exactly alive like we are. They need to hijack other living organisms in order to reproduce. And this is their only goal. To survive and replicate. A virus replicates only inside the living cells of an organism. 

When infected, a host cell is forced to rapidly-produce thousands of identical copies of the original virus. The Process of replication is explained in the below graphic.

Source: Lumen Learning

Viral infections in animals provoke an immune response that usually eliminates the infecting virus. Immune responses can also be produced by vaccines, which confer an artificially acquired immunity to the specific viral infection. Some viruses, including those that cause AIDS, HPV infection, and viral hepatitis, evade these immune responses and result in chronic infections.

The current pandemic caused by the Corona Virus is also an influenza flu virus which functions in the similar manner. There are numerous possible ways in which a virus can evolve to cause an outbreak in humans. Recently, you may have heard this a lot that it’s a bat virus and that it jumped from Animals to Humans. This is one of the ways how a virus can enter humans. Let’s understand how exactly does this jump occurs!

A zoonosis is an infectious disease caused by a pathogen that has jumped from non-human animals to humans. Major modern diseases such as Ebola virus disease and Salmonellosis are zoonoses.

HIV was a zoonotic disease transmitted to humans in the early part of the 20th century, though it has now mutated to a separate human-only disease.

Source: CDC

Most strains of influenza that infect humans are human diseases, although many strains of bird flu and swine flu are zoonoses.

These viruses occasionally recombine with human strains of the flu and can cause pandemics such as the 1918 Spanish flu or the 2009 swine flu.

Some of the virus present in animals are similar to flu virus prevalent in human population. These virus have a high probability of transmission to humans and likely cause an outbreak. It generally happens in two manners. Either the virus mutates in the host species itself. Then the virus can spill over to the humans through various modes. See the Pic Below.

Modes of Zoonoses
Source: FoodSource

Another manner in which virus can evolve is through intermediary species.
An animal virus from a non human species, and a flu virus from humans when meet in a common species, it evolves to form a deadly virus capable of causing a pandemic in humans.
This is a matter of chance.
And this chance event is what happened on a Kansas Farm in 1918.
Although experts aren't sure but they believe that 1918 Spanish Flu Pandemic could have started when a bird with the flu and human with the flu met the same pig.
Before that, bird flu could not infect humans and human flu could not affect birds.
But both of these virus combined inside a pig cell to create a new zoonotic virus
H1N1. This new virus could definitely infect humans.

In the next Episode, we will take a look at how a flu virus spreads in humans, its phases of transmission, and list down world's worst pandemics hitherto.

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