What is a virus?
A virus basically consists of genetic material that resides inside an envelope.
The genetic material is either DNA or RNA. The envelope consists of proteins, lipids, or both. Some viruses also carry some enzymes inside their envelope. For example, some RNA viruses contain an enzyme (reverse-transcriptase) that converts the virus’ RNA into DNA, after which the DNA takes over command of the infected cell.
This description is simplified. Much more can be said about viruses, their construction and their workings. The above description suffices for the purposes of this article. Those who like to know more can read the excellent Wikipedia page about viruses.
What is the difference between viruses and bacteria?
Bacteria are living cells. They have machinery to metabolise a form of food, they sense and react to their environment, and they replicate.
Viruses are basically nothing but DNA or RNA in an envelope. They do not live. They don’t digest anything, they don’t sense their environment, they even don’t replicate.
This is why bacteria can be killed by antibiotics, but viruses cannot. Antibiotics disturb one or more processes that are essential to the bacteria’s survival. Viruses do not exhibit any process at all, so there is nothing to disturb.
Then how does a virus infect a cell?
Again, all details are explained on the Wikipedia page about viruses. I will only give a simplified explanation here, as that will suffice for the remainder of this article.
A virus may infect a cell by an automatic chemical reaction between a protein in its envelope, and a protein on the cell membrane. Depending on the virus, this chemical reaction may result in the fusion of the virus envelope with the cell membrane, which results in the release of the virus’ contents (DNA or RNA, and possibly some enzymes) into the cell. Or the chemical reaction results in a temporary hole in the cell membrane, through which the virus’ contents then quickly enter the cell.
What happens after infection?
After infection, the cell’s machinery does what it always does: it transcribes (“copies”) DNA into RNA, and it builds proteins according to the instructions on the RNA.
After an RNA virus enters the cell, the RNA is translated directly into proteins by the cell’s machinery. If these proteins are RNA-copying proteins, the RNA is also copied many times.
If the virus is a DNA virus, the DNA is copied to RNA, and the same process as described above takes place.
If the virus is an RNA virus with the reverse-transcriptase enzyme, the enzyme makes a DNA copy of the virus RNA. Then, the same process occurs as in DNA viruses.
In all cases, the end result is that the normal machinery of the cell starts executing the instructions that come from the virus RNA/DNA. These instructions tell the cell to create lots and lots of copies of the virus RNA/DNA, and to create lots and lots of the proteins that are in the virus envelope.
The copies of the virus RNA/DNA and the envelope proteins then self-assemble into new virus particles. Alternatively, some viruses instruct the cell to produce enzymes that take care of the assembly of new virus particles. In both cases, the cell fills up with new virus particles. The new virus particles are either excreted by the cell, or they fill up the cell until the cell bursts and the particles are set free.
The end result is that the original virus particle that infected the cell no longer exists, but now there are many thousands of identical virus particles.
The body’s lines of defense
The first line of defense
Normally (that is: when we are not wounded), viruses can only enter the body through one of the mucous membranes. Normally, this will be a membrane of the nose, mouth, or throat.
A virus, being a passive construction, cannot actively swim through the mucus in the direction of a cell. Instead, it just sits in the mucus, waiting until chance brings it in contact with a cell.
The body contains intrusion-detecting cells and enzymes that are also present in the mucous linings. If these encounter a virus, they will destroy it. This prevents the virus from entering the body at all.
The second line of defense
A virus that is not destroyed by the first line of defense may eventually reach one of the cells of the mucous membrane. It has to infect these cells in order to enter the body. The mucus cells are highly resilient to virus infection. If infection does occur, they quickly excrete alarm proteins that alert the body to the infection.
(Note: I’m not an expert in this field. Therefore, the above description may be overly simplistic, and it may even be a little off. If you have more knowledge about this second line of defense than I have, please share your knowledge, so I can improve this part of the text.)
The third line of defense
Once the virus makes it through the layer of cells that line the mucous membrane, it is inside the body. The body now depends on its third line of defense to fight the virus. This third line of defense attacks the virus in two ways.
The first possibility is that the body attacks the virus itself. This can only be done with virus particles that have not yet infected a cell. These virus particles are recognised by the proteins in their envelope, which are not human proteins. The virus particles are marked by special markers that have two sides: one side chemically binds to the virus protein, the other side signals immune cells to “kill me”. The immune cells then absorb the markers and the viruses to which these markers are attached into little intracellular bags called lysosomes. The cell then fills the lysosome with destructive enzymes. This kills the virus, reducing it to recyclable amino acids and lipides.
The second possibility is that the body attacks the cells that are already infected by the virus. These cells are doomed anyway, so they may as well be killed immediately. Cells that are infected by a virus mark themselves by embedding specific proteins in their outer membrane. These proteins are recognised by markers in much the same way as the virus proteins are recognised by markers, as described above. The cell is then killed, also killing all viruses inside it.