In biology, the term obligate refers to a species which is dependent on a specific niche or circumstance in order to survive. All viruses fit that description, along with a few bacteria. None can multiply or perform other functions outside of a host. Luckily for them, there are plentiful hosts available. Viruses can infect human, animal, insect and bacterial DNA.
Viruses are not technically alive but exist as a molecular package of RNA or DNA, coated in protein. For example, the influenza virus is a RNA virus, while the HIV virus is composed of DNA. The word ‘virus’ is Latin, meaning toxin or poison. Viruses have been described as “organisms at the edge of life”. Requiring the functioning cell-life of the host, a virus infects by various means and hijacks the DNA replication machinery of a cell. There, the virus uses the enzymes, ribosomes, nucleotides and amino acids made newly available to synthesize its own reproduction. Viral packages bud off from the hostage host cell as the virus replicates itself.
For most of us, primary concern is immediate protection from known scourges. It is the reason we take our children for their vaccinations, the reason we avoid those who are coughing, sneezing or otherwise obviously ill, the reason we are cautioned to use a condom, the reason we are exhorted to wash our hands. Humans routinely fight the flu, the common cold (RNA viruses) and are exposed occasionally to herpes virus, Epstein–Barr virus and HIV (DNA viruses) among many others. Elsewhere in the world, measles, mumps, rubella and chickenpox are too familiar and endless battles. An entire recitation of viral illness would fill several books.
Viruses live in various body fluids, each with its own rules of engagement. They vary in their level and length of threat to our health based on immune response, previous exposure, and general health. As a social species, humans are voluntarily and involuntarily exposed to the body fluids of others on a regular basis. Our immune system defends against viruses and other tiny attackers every moment of our lives. It is important to our survival and has evolved in a microscopically hostile world.
Research has identified many applications for exploiting this viral proclivity to hijack and reproduce. It is, indeed, a brave new world; one that can’t help but hold the imagination. A genetically modified virus has been used to create a more environmentally friendly lithium-ion battery. Modified viruses have been successfully used in gene therapy in the treatment of certain genetic disorders and research is continuing in the application for cancer therapy.
It is theorized that our own mitochondria might once have been an obligate virus. Might we be the winner of the Survivor episode of the eons?
Bruce Heyer DeAnza College
Dr S Selkirk Department of Neurology, University Hospital of Cleveland
Eugene V Koonin, National Center for Biotechnology Information, National Library of Medicine, USA
Tatiana G Senkevich, Laboratory of Viral Diseases, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, MD 20894, USA
Valerian V Dolja Department of Botany and Plant Pathology and Center for Genome Research and Biocomputing, Oregon State University, Corvallis, OR 97331, USA