Viruses are really pieces of genetic information in the form of DNA or RNA enveloped in a protein casing and sometimes a membrane picked up from animal cells. Influenza is no exception to that. It consists of eight segments of RNA surrounded by a protein shell and a membrane envelope. H5N1 actually denotes the types of proteins found in the envelope.
All types of influenza carry H (haemagglutinin) and N (neuraminidase) proteins on their surface. Haemagglutinin is required for entry into animal cells as this is the protein that binds to a molecule called sialic acid on the cells and tricks the cells into taking the virus inside. Once inside, the virus takes over the cell and makes thousands of copies of itself that are released into the body from the infected cell.
Neuraminidase is the protein required for the virus to be able to leave the infected cell. It destroys sialic acid on the surface of the cell that has become a virus factory so that, when new viruses bud from the surface, they do not get stuck to the infected cell by binding with haemagglutinin. Neuraminidase makes sure there is nothing for the new viruses to bind to on the already used cell, so the new virus particles can spread to other cells and infect them.
There are several different types of H and N proteins, and H5 and N1 are just some of them. Our bodies have seen combinations like H3N2 recently but humans have not encountered H5N1 before, which is why it is dangerous. Because our immune systems have never come across these particular proteins, we have no prepared defence against them. Thus, this virus could cause a huge epidemic if it started to spread between humans.
Why can’t it do it now? The virus is avian and it is adapted for replication in bird cells, so at the moment it is not a danger. All of this can change very quickly, however. All it requires is a bird that is simultaneously infected with a human influenza virus and H5N1 avian virus and a bit of ‘luck’ for the virus. In this case, the eight segments of the two viruses would be inside the same animal cell, and they could exchange segments with each other. If H5N1 picks up the human virus segments required for effective transmission between humans, then we have serious trouble on our hands. Influenza spreads very easily by inhalation of infective virus particles or picking them up from surfaces, so this method of virus spread combined with global travel could rapidly take the virus to anywhere in the world.
The key point is that we don’t know whether it is going to happen or not. The element of chance there is unpredictable. However, we can’t just sit around and wait for the virus to become a serious problem. The last epidemic during the First World War killed 40 million people. We need to take serious measures to prevent the possibility of this virus causing worse casualties, such as having production of a vaccine arranged at the first signs of spread between humans. Many governments are stockpiling Tamiflu, which is the only effective drug we have for influenza so far. The problem that will arise very quickly, however, is resistance to this drug as influenza mutates at a very high rate. We cannot just rely on this one drug as the magic bullet because it is not. Vaccine measures need to be in place as well. Education about preventing virus spread, even simple things like washing your hands very regularly, could also help.