Influenza invading the lung

Dear Minister Hogan: You would know that, unlike bacteria, viruses are not free-living organisms. They need the cells of other organisms like ours to reproduce. 'They are actually lifeless, inert chemical particles' John Farndon wrote in Bird Flu, but 'once they get inside a living cell, however, they change completely, taking it over like some demented house guest.'

There are no other forms of life like a virus. They have no cell. They do not have the cell structures for eating and the making of energy. All they can do is copy themselves and even that they cannot do on their own. Viruses are very bad news wrapped in a protein.

David Baltimore gives the following description: 'A virus is really nothing more than an alternative set of instructions that can take over the cell and make the cell into a virus-producing factory. As in our own DNA, the instructions are broken up into genes. Because viruses put such a premium on being small, the virus's genes are notably compact but also notably changeable. The rapid change means that a virus can readily adapt itself to new circumstances.'

Retroviruses like HIV, as the name suggests, consists of ribonucleic acid or RNA and, along with it, an enzyme called reverse transcriptase. David Baltimore (at MIT at the time, in 1975) and the late Howard Temin (at the University of Wisconsin at Madison at the time) first described reverse transcriptase before HIV made its pandemic appearance.

Once the HI virus breaks into the cell (it does this by finding a host cell with chemical receptors that provide a perfect match for its protein and, sometimes, a fat-based coat) reverse transcriptase interrupt the normal process and converts viral RNA into a DNA copy.

It is normal for DNA to convert to RNA as a prelude to the making of proteins. Reverse transcriptase changes the process with the consequence that the viral DNA is integrated into the cellular host and hijacks the genetic machinery to make more of itself in their thousands and millions.

The RNA sequences of HIV1 and HIV2 were described in the 1990s. HIV's genome consists of a ribonucleic acid or RNA molecule that is host to at least three genes, where one called gag encodes the core proteins, pol encodes the viral enzymes and env the envelope glyco-proteins.

HIV belongs to a family of viruses called lentiviruses. The other members of the family include the simian (monkey) virus or SIV, the feline (cat) virus or FIV, bovine (cow) virus or BIV, equine (horse) infectious anaemia virus or EIAV and, finally, the Visna Virus which cause disease among sheep.

SIV is the source of HIV, with different versions of it being linked to its two variants HIV-1 and HIV-2. It also appears that they entered human populations separately and at different times.

It is unclear when the species barrier was crossed. HIV-1 has been isolated in Zaire in 1959 and Burkina-Faso in 1963. HIV-1 has been in human populations for at least 40 years. The possibility remains that the viruses have been introduced into human populations many times in the past but failed to become epidemic. Infections either died out because of the low trans-missibility of the virus or it smouldered on in a small fraction of the population.

Recent changes in behaviour, including more extensive travel by truck, bus and plane, sexual practises, the use of injectable drugs, as well as the increase in human population, may have allowed the virus to spread more extensively than in the past and to become epidemic worldwide.

HIV's primary target of attack is the one arm of our adaptive immune system, what is known as CD4+ T cells. These cells are a crucial part of the host's adaptive immune system, which reacts to the pathogen vigorously, at first with monumental antibody production, but after a considerable but varying period of latency lasting up to 6 years and where the virus effectively goes into hiding, the immune system is overwhelmed, HIV replication escalates, opportunistic infections occur, cancers appear and neurological symptoms become evident.

There is no prophylactic vaccine in existence and Baltimore reckons we will not see one for 20 years, if at all. Under the circumstances the best vaccine to prevent new infections is a monumental education effort and your political leadership.