Viruses are the simplest form of life. Its their simpicity that causes the debate on whether they are alive or not. Thus it is my understanding that a strain of bird flu implies that the DNA sequence is very similar to other strains i.e. it is the EXACT sequence of DNA that specifies the strain. If a mutation occurs such that even one base changes then this becomes a new strain. So how could the bird flu at BM be described as H5N1 prior to today's expected confirmation? Any one able to supply the answer?

The only way a virus can reproduce is by invading a cell in a host and causing that cell to spend its energy reproducing copies of the virus. This causes the host to fight the virus. The virus only survives as a species if it is transmitted to another host. The virus can either do little harm to the host and use the host to expel it, in some way, giving it a chance to get to another host. Or it does a lot of harm, killing off the host. The virus then has to take a chance that it can survive outside a host until it gets absorbed into another suitable host. The common cold is a virus that does little harm and spreads rapidly. Ebola does a lot of harm, more often than not killing off the host (at least in humans). Human Immunedefficiency Virus (HIV) comes somewhere in between.

The only reason we know that some viruses need bodily contact to move from host to host is empirecal evidence. We do not know why one virus can survive a long time outside a host while another one can't. We do not know why a particular virus attacks a particular type of host (e.g. birds) but not another (e.g. humans); we do not know the characteristics or scientific reason. In other words, we can identify a virus but we cannot explain why it moves from host to host only via bodily contact or only through the air (colds).

Viruses multiply very rapidly and are subject to Darwinian evolution meaning that over time they change. Sometimes a change will result in a virus being able to attack a new host. This is termed species jumping. Since we do not understand why a particular virus attacks a particular host we only find out about a species jump when another type of host develops symptoms. Despite all these unknowns viruses are used in creating genetically modified crops.
We only have one defense against viruses. That is our own immune system, which works well against most types of viruses (but is still not well understood). We enhance our immune system's capabilities by innoculations. These give our immune system a practice session, so that if we are invaded our immune system is ready to repell invaders.

Using electron microscopes we can see what a virus looks like. We can take a virus apart and chart, or sequence, its DNA. The DNA of the bird flu in the UK was eventually reported as being 99.6% the same as that in Hungary. Before this the UK bird flu was identified as the type, or strain, called H5N1. What are the characteristics that allowed it to be identified as H5N1? Was it its DNA sequence? By how much must the DNA change before it gets described as a different strain?

Are the descriptions and assumptions about viruses corerect and up to date?