Almost every health scare these days seems to concern viruses. From bird flu and Ebola and now to Zika, these pathogens appear to have medicine on the hop. But what exactly is a virus, and why are viruses such a problem?
A typical virus is a remarkably simple machine. It is just a short stretch of nucleic acid (DNA or RNA) surrounded by a protein coat. The nucleic acid contains coded information for making new virus particles, while the protein coat may help the virus gain access to its host. And that is that. Viruses have no membranes, no complicated machinery for carrying out reactions, and no metabolism like one of your own cells. Viruses do not feed, move, or respond to their surroundings like proper organisms. And they are so small that they were not even visible until 1939, following the development of the electron microscope.
However, introduce a virus into a host cell and the results are dramatic. It hijacks the cell’s processes and redirects them exclusively to the manufacture of more of itself. New virus particles are then budded-off from the surface, each surrounded by a piece of host cell membrane. Or, the cell splits open, releasing hundreds of new particles to infect other cells. Either way, viruses damage and kill our cells, which is what makes us ill when we have a viral infection.
Another trick of viruses provides a hint as to their origins. The genes in our own cells are remarkably like virus particles, also consisting of nucleic acid (DNA in this case) surrounded by protein. Sometimes a virus splices itself into this host DNA like an extra gene. The virus then lies low, being copied with the rest of the DNA when the cell divides, and passing to each of the daughter cells produced. Later, it may emerge without warning to form more viruses in the normal way, causing illness years after it first invaded. This is what happens when the chicken pox virus causes shingles in later life, or when people recovered from Ebola relapse, as recently happened to the Scottish nurse Pauline Cafferkey.
So, if viruses are constructed and can behave just like normal genes, perhaps that’s what they really are? Perhaps they are “escapee genes” that left their cells long ago to take up an independent existence? That would explain why they find it so easy to invade and take over our cells, and why we find it so difficult to defend against them.
Whatever the truth of their ancient origin, viruses present modern medicine with a formidable challenge. Antibiotics do not work against them, and the particles mutate rapidly to keep ahead of vaccines prepared to defeat them. One thing is certain: Ebola and Zika are not the last health scares that they will bring us.
Viruses and the diseases that they cause, including ‘flu and Ebola, are covered in depth by the new A Level Biology course recently launched by Oxford Open Learning. You can find out more about the course here: http://www.ool.co.uk/subject/a-level-biology/
Philip West studied Natural Sciences at Cambridge and taught for many years in Yorkshire and London. He is now a writer, editor and tutor, and also an examiner for Cambridge International Examinations. He is the course writer for all of the Oxford Open Learning science courses, including the brand new A Level Biology course.