Wellcome trust/Guardian science writing competition entry

Viruses: Friend and Foe

A sniffle and a sneeze, maybe some vomit on the pillow and a seemingly innocuous fever. Or, a pool of blood haemorrhaged from your precious organs.  Waking up with any of these symptoms is going to put a crimp on your day, most notably the latter, but all of these are potentially caused by viral infections. However not all viruses offer up such conclusive evidence to their presence, and why do their symptoms range from the benign to the sensational? Take for example HIV, which has two common strains in humans: HIV-1 and HIV-2. Untreated HIV-1 eventually leads to AIDS (acquired immunodeficiency syndrome), whereas HIV-2 does not have the same devastating impact on the immune system, fulfilling the role of a quiet “keeps themselves to themselves” lodger. Even HIV-1 will not progress to AIDS for up to 20 years in some infected individuals.

Charles Darwin, the veritable godfather of evolution based his hypothesis on fossilised bones of animal and plants, but the infinitely successful theory is recapitulated in all life forms, including viruses. Mankind’s’ evolution began from a single celled organism in which natural mutations over the years gave rise to characteristics in offspring which increased their survival. These mutants bred, and their offspring which also harboured beneficial mutations survived and again passed on their genes, and so on… This took a while, in fact over a billion years, until homo finally became erectus.

Virus evolution is also based upon survival of the fittest. In this case the goal is the same - stay alive, passing on beneficial genes to progeny virus. This is however a little closer to our hearts (literally) as “we” provide the environmental pressures which funnel the survival of those viruses capable of replicating. By “we” I mean our immune system, and viruses evading our defences can successfully spread. From our side of things, if we don’t suffer from the infection then we are none the worse, and the virus has successfully evolved to go undetected.

Ebola Virus (Transmission Electron Micrograph)

Viruses which cause drastic symptoms akin to generic horror story scenes - like Ebola - occupy an interesting niche. They haven’t evolved to go undetected but instead head down the opposite route: evading irradication or prolonging it long enough to spread to the next individual. In the case of Ebola, rapid multiplication results in host organs literally bursting with virions, unfortunately for these progeny they’re left without a host – usually prostrate on a hospital waiting-room floor amid screams. This ghastly eruption aids the spread of Ebola as people come into contact with infected blood, and to increase the chances of transmission the more spectacular and far reaching the haemorrhage the better. A less extreme example of another non-evasive virus is influenza, which maintains its infective stronghold throughout the world due to its airborne transmission and peak infectivity prior to the onset of more severe symptoms.

Some viruses however do go undetected, and many retroviruses have incorporated themselves into our genome. In fact 40% of our DNA is comprised of retroviral DNA, which as we’ve co-evolved with viruses has meant that humans with beneficial retroviral DNA incorporation survive longer and transfer the genes to their offspring (namely us). Viral infection has therefore aided our evolution, providing genes which have now been found to be cruical for antibody production and countless enzymes. Without the helping hand provided to humans via incorporation of viral DNA, our evolution would be pushed back a million years or so.

Another mechanism which some viruses use to go undetected is through mimicking human proteins. This allows a virus to go undetected by the immune system which recognises the virus as 'self', and therefore does not elicit an anti-pathogen clearance response. This mimicking isn’t necessarily reflected in the functional aspect of the protein mimicked by the virus, but how the virus looks in a 3D form, and fundamentally how the immune system recognises them.

Our body is designed not to mount an immune response against our own self proteins but will against bacteria, virus or transplanted tissues. This coaching of our immune system to tolerate our own tissues is neatly enough called “tolerance”. A break down in tolerance occurs when the body attacks “self” in what are classified as autoimmune diseases (type 1 diabetes, crohn’s, rheumatoid arthritis etc…). Viruses which invade a host with similar “looking” proteins to our own can inadvertently kick start self-reactive immune cells previously put into retirement by our body through tolerance, triggering autoimmunity.

Exploitation of virus can also aid mankind, for example viruses which infect and destroy bacteria (bacteriophages) may become the next generation of anti-bacterial agents (see video below). Research into bacteriophages is becoming even more crucial as the number of bacterial strains resistance to antibiotics increases, compounded by the fact that many treatments which knock out our immune system (radiotherapy in some cancers) rely on long term antibiotics to protect patients from infections.

Viruses conjure up images of gruesome deaths which can occasionally be the case, but less sensational infections may also be as harmful. Why not then be grateful for the occasional annoying summer cold for its respective benevolence? On the other hand we should embrace viruses for speeding up our evolution, and as our potential future saviours against bacterial infections. Viruses are Janus-like two-faced Gods of reality, with the power to cause the end of man, or provide the beginnings to our continued evolution and fight against bacterial infection.