Another key part of the definition of a virus is that they're small.
And years ago it use to be very small
but we've change this as we discover new viruses
because we're finding bigger and bigger viruses.
So, let's illustrate that by this slide.
Here we have a part of an E.coli bacteria
which a rod-like bacteria that are part of your intestinal tract.
It's magnified about a 100,000 times.
You can see some flagella on this slide.
And attached to the bacterium is a bacteriophage or a virus that infects the bacterium.
You can see it's quite small compare to the bacterium itself.
Now, above the bacterium that first green line is a virus particle.
It's called tobacco mosaic virus. It's a virus of plants.
It's the first virus that was discovered.
And above that to the left its an HIV-1 virus particle.
So you can see in relation to the bacteria they're quite small
although they are visible at this magnification.
Now, in the little box there are other viruses as well as cellular components that I'd like to compare.
So you get an idea of how big things are, but they're too small to see in this magnification.
So, let's move up to a million fold magnification and now we can see them.
At the bottom on the left is a poliovirus particle, and above it is a ribosomes.
So, a ribosome is cellular component that's used to make protein.
So, you can see polio is about the size of a ribosome.
And all the way above it the component mark A that is a carbon atom.
So, you can see viruses are on the atomic level.
Some of them are not much bigger than individual atoms.
Now, to the right of this slide are various cellular components.
They're acting in miles and fibrous in a variety of enzymes of different sorts.
So, viruses exists at the molecular level but they also get much bigger as well.
Here's another way of looking at the relationship between virus size and cell size.
Here is a cell surrounded by four virus particles.
These happen to be herpesviruses.
And let's expand this and look at just the part of the membrane of the cell.
You can see the plasma membrane and then beneath it are a variety of cellular components like ribosomes.
And there's the herpesvirus particle which is 200 nanometer in diameter.
Where you can also see now at this magnification there's some polioviruses
which happen to be about 10 times smaller than the herpesvirus.
So, we couldn't see them in the original picture.
So, viruses are much smaller in most cases than cells.
Maybe some of you are wondering how many viruses would fit on the head of a pin.
So, let's answer that question.
Here is a pin head and in the very center you can see something that's very tiny and red.
So, let's expand that. That happens to be a red blood cell.
Now, what we could not see in the original pin but we can see now.
There's something green to the left of the red blood cell.
What is that? Those are E.coli cells.
And now, at this magnification we can see there's something below the E.coli.
Then if we magnify that even more we can see that's Ebolavirus.
And below the Ebolavirus now is a Rhinovirus.
So, the only thing we can see on the pin in the very first magnification is the red blood cell,
but you can see that viruses are much smaller.
So, the answer is about 500 million Rhinoviruses will fit on the head of a single pin.
That's a lot of viruses and when you sneeze you're exhaling droplets of viruses.
If you happen to have a common cold.
So, if you're infected with a Rhinovirus every time you sneeze
you're firing aerosol that contains thousands and thousands of viruses.
And this of course is part of the viral strategy for finding new host.
They make lots of progeny, most of them don't go anywhere
but you just need one to find a new host and start a new infection.