Viruses are entities at the limit of living and non-living. Unlike bacteria, a virus can not feed and reproduce by itself; instead, it needs a host cell to accomplish these functions.
That's why they induce diseases; over 50 % of the human diseases and 90 % of the human infections are virus-provoked. A virus is made by a molecule carrying its genetic information (DNA or RNA) and a protein coating (in some cases). They lack crucial elements for an autonomous life, like ribosomes or cell membrane.
A virus penetrates inside the targeted host cell, and when it reaches its nucleus, this stops working on its own DNA and starts to produce copies of the viral DNA.
In case of RNA viruses, their RNA is turned into DNA and that DNA produces multiple copies of the viral RNA. A host cell can produce till 500 viral copies till it breaks out or simply dies off.
A virus will enter a human body through small skin or membrane (genital or mouth) lesions (like HIV, herpes virus or common wart virus), food (hepatitis A) or droplets (flu virus). Initially, a virus infects the penetration area, but after a time, it reaches all the body through the blood and lymph.
But not all the infected ones get the disease. Only 50 % of those infected by the measles virus suffer by this disease, and in the case of the poliomyelitis virus the rate is even lower: just 1-10 % of the infected develop polio.
Normally, children, those already ill, elder persons and pregnant women are more sensitive to viral attacks, but some viruses can prefer young and healthy people, like that of the Spanish flue, which killed 20-40 million persons in 1918-1919.
Highly contagious viruses must be avoided through quarantine. Another method of fighting against viruses is the vaccine. A healthy person receives a dose of deactivated virus, virus bits, or weakened viruses. The body will react as in the case of a real virus, and will produce specific proteins called antibodies.
If after that the organism enters in contact with the real virus, it already has prepared antibodies and immune cells and will begin an immune action. The viruses die soon and the person will not manifest any symptom. It must be mentioned that antibodies fight only against the viruses that the organism already knows (by vaccine or through inherited genetic information). Breathing masks and gloves can also help in case of epidemics (like SARS).
There is a difference between the healthy cells and the infected ones. That's why in the case of some viruses, researchers were able to develop drugs destroying the ill cells and at the same moment the viruses they carry, like in the case of herpes or hepatitis.
You must know that antibiotics attack just the bacteria; they are of no use against the viruses.
Along their evolution, people developed special immune cells, called killer cells, that locate and eliminate the infected cells, sometimes before the virus reach reproduction.
At the same moment, there are chemicals (interferon and interleukins) that alert the neighboring cells, still healthy, of the closeness of the virus.
Other barriers are represented by the special proteins of the mucosa membranes or the secretions of the respiratory tract, which can retain, for example, the flu virus. Fever is another protective factor: at temperatures that are too high, many viruses cannot multiply.
How can viruses avoid the body's protection?
By continuous mutation.
As its DNA is constantly changing, the human immune system can no longer recognize the typical viral structure. Some viruses modify so fast that they outrun body's defensive.
Usually, the viruses have a specific host (they live in just one or few species to which they are adapted).
But those with a high mutational process are able to jump to another species and produce the disease, and that's the way many new viruses appeared in humans, transmitted from animals, like HIV or SARS.
People can take viruses from animals by eating or touching infected animals or their secretions or through the air. This is favored when people and animals live together in small spaces, like in the case of SARS in south Chinese markets or by bush meat, the hunt in pristine places, teaming with new viruses that can jump to humans (and against which humans are defenseless), like it happened in the case of HIV.
When animal viruses enter in contact with human viruses (like flu virus) that transmit easily, and interchange DNA portions, can result new extremely powerful viruses that can inflict massive epidemics, like in the case of the Spanish flue.
That's the concern about the bird flue virus: whether or not it will interchange DNA material with the common human flue virus.