VIRUSES are truly remarkable. The organisms which give us HIV and rabies, polio and malaria, and indeed the deadly flu (which killed more in 1918 than World War I) are themselves barely alive.
A virus is a single strand of DNA. It is a strip of the nucleic acid (deoxyribonucleic acid) which is present in all genes. It is no more than a short string of genetic information, enclosed in a skin of protein.
And thus, it occupies the borderline between our ideas of the living and the non-living.
Viruses don’t eat, breathe or excrete. (Even bacteria manifest those characteristics of life.)
Indeed, viruses can become crystallised and stored for years in a bottle like inert crystals of sugar or salt. Immediately you put them back in a host environment, however, the virus emerges once again as a “living” organism.
The closest analogy is that of a computer virus. Comprising a string of letters and numbers, a computer virus can wreck computer programs. It can arrange its reproduction and transfer into other computers. But it’s still just a string of letters, a code which can be written on a piece of paper or even memorised. But key it into a computer program, and immediately it begins to do whatever it was programmed to do.
Dr WM Semper, who won the 1946 Nobel Prize for chemistry for his work on viruses, demonstrated the “non-living” quality of a virus, which he crystallised from plants infected with Tobacco Mosaic disease. So viruses don’t only affect humans. The ongoing Foot and Mouth Disease pandemic, which affects hooved animals, is caused by a virus. Even bacteria, can be host to a virus.
The first viruses which originated in the sludge of the primordial seas on the Earth, could be thought of as “naked genes”. All they comprised was this DNA strand. Later forms, however, were protected by a protein coat, which sometimes has appendages with which it attacks neighbouring cells.
Unlike other simple life forms, they did not develop a nucleus. All they do is reproduce themselves, and only inside a host cell.
The viruses which attack bacteria have a head and a tail. The tail attaches itself to the bacterium while the head penetrates the cell wall. Other viruses are simpler in shape but they all penetrate the cells of their chosen host. Once inside they take over its activity, substituting their genetic programming for that of the invaded cell.
Much as a computer virus operates, the living virus reprograms the cells of its host to turn it into a virus factory. New viruses are thus produced by the host’s own cell.
Viruses are responsible for human diseases, which include herpes, hepatitis, influenza, measles, polio, rabies, rubella, smallpox, yellow fever and a host of others including HIV. They are transmitted from one victim to the next by direct or indirect contact.
How that is accomplished is as amazing as anything else about viruses.
Imagine something as simple as a piece of genetic information, a list of letters slightly longer than this article, which has no specialised cells, yet it can carry out an action plan of surprising cunning.
Cold and flu viruses position themselves in your saliva and then make you sneeze. That’s simple enough, but what about rabies? The rabies virus enters the nerve endings of its host, and works its way up to the brain. Days, months, even years later, it causes encephalitis, driving the victim violently mad. Only then does the virus migrate to the salivary glands, so when the infected animal in its rage bites another creature, the virus is transferred in the saliva.
Herpes hides from our immune system by entering the nerve cells, occasionally emerging briefly to create sores in the mouth and genitals, thus infecting other hosts through kissing and sex.
Herpes has been around for millions of years, partly because it doesn’t kill its host. The Ebola virus, which broke out in 1976 in Zaire, is of far greater savagery. Spread through blood and sex, it has no symptoms for the first few days. Within three weeks, however, the infected person develops muscle pains, headaches and fever. The virus then destroys the lining of the blood vessels, and the patient quickly dies of internal bleeding accompanied by vomiting and diarrhoea.
Such virulence is comparatively easy to identify and contain. HIV, which is more slow-acting, is more insidious. Imagine what would happen if it could be transferred by a sneeze! Vaccines consisting of dead or weakened viruses are the best protection against viral diseases and they have been responsible for the virtual eradication of diseases like smallpox and polio, which were once common. The injection of the vaccine results in the production of anti-bodies, which can provide protection against a specific disease for many years.
It was long observed that people rarely got second attacks of smallpox or measles, and indeed a form of vaccination was practiced in Turkey centuries ago.
In the early 18th century Caroline, the Princess of Wales, heard about the Turkish practice of inoculating children with smallpox. She tested it on six condemned prisoners, and then on 12 orphans, before inoculating the young princesses in 1723.
Forty years later Sir Jeffrey Amherst, the British military commander in North America, distributed blankets contaminated with smallpox to American Indians who made life difficult for settlers in Pennsylvania.
Thus viral warfare was invented before Dr Edward Jenner injected the cowpox virus into a living patient in 1798, despite the scepticism of his peers, and thus vaccinated his patient against the closely related smallpox virus.
Jenner’s experiment made medical history. Today finds us still battling viruses, most notably the HIV virus, and all the while still making other strains with which to wage war. As if we’d never learnt a thing.