Hello everyone, welcome to our immunology club. 
In this video series, we explain basic biology of immunology and virus infection without using technical terms. 
Today, we will talk about how virus expand in our body and go on to a journey outside our body. 
Last time, we talked about how virus expand within a cell and then exit the cell. 
When newly copied virus trying to escape from the cell, infected cell might collapse if there are too many virus. 
The next destination of the virus is the healthy cells around who hold the same sensor with the specific keyhole. 
By repeating the process of infecting, proliferating and escaping from the next cell,  
virus undergoes crazy expansion by damaging a lot of surrounding cells. 
By the way, do you know which organ are we observing in our body? 
Right, it’s lung and trachea. 
These two organs are responsible for a very important function for us to keep alive, that is, breath. 
Of course, during the virus infection and proliferation, our body will never silently watch aside. 
Here welcome our hero and guardian, immune cells. 
The great war with the virus is about to start! 
Immune cells are in charge of cleaning up the dead cells, signal transduction to other cells, spying on the enemies and fighting with the virus. 
During this fierce fight, of course a lot of cells including immune cells would die,  
which stimulate the secretion of sputum to clean and flush away the dead bodies and makes one hard to breath and start coughing. 
Within the sputum and spit, there is tremendous amount of virus being kicked out of body after crazy expansion. 
However, this is not the end of the virus big dream. They still want to proliferate. 
Next time, we will talk about how virus get out of our body and further level up their expansion. 
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