Viruses

Introduction

That's the salubrious thing about zoonotic diseases: they remind us, as Saint Francis did, that we humans are inseparable from the natural world. In fact, there is no "natural world", its a bad and artificial phrase. There is only the world. Humankind is part of that world, as are the ebola viruses, as are the influenzas and the HIVs, as are Marburg and Nipah and SARS, as are chimpanzees and palm civets and Egyptian fruit bats, as is the next murderous virus - the one we haven't yet discovered.

No one knows exactly when viruses emerged or from where they came, since viruses do not leave physical evidence in the form of fossils. Modern viruses are thought to be a mosaic of bits and pieces of nucleic acids picked up from various sources along their respective evolutionary paths. Viruses are acellular, parasitic entities that are not classified within any of the three domains because they are not exactly alive. But they do parasitize, evolve, reproduce and co-evolve with other organisms; they inhabit a shadowy world that may not be alive, but is very close to it. They have no plasma membrane, internal organelles, or metabolic processes, and they do not divide. Instead, they infect a host cell and use the host’s replication processes to produce progeny virus particles. Viruses infect all forms of organisms including bacteria, archaea, fungi, plants, and animals.

Viruses are diverse. They vary in their structure, their replication methods, and in their target hosts or even host cells. They infect every type of organism known, from Archaea to Bacteria to Eukaryotes, and are found in every environment. They are also remarkably abundant; it is estimated that each milliliter of sea water contains 10 ⁷ viruses, both DNA and RNA varieties. They are major players in the evolution of the life forms on this planet; genes derived from viruses allowed mammals to develop a placenta, for example.

How viruses replicate

Viruses were first discovered after the development of a porcelain filter, called the Chamberland-Pasteur filter, which could remove all bacteria visible under the microscope from any liquid sample. In 1886, Adolph Meyer demonstrated that a disease of tobacco plants, tobacco mosaic disease, could be transferred from a diseased plant to a healthy one through liquid plant extracts. In 1892, Dmitri Ivanowski showed that this disease could be transmitted in this way even after the Chamberland-Pasteur filter had removed all viable bacteria from the extract. Still, it was many years before it was proven that these “filterable” infectious agents were not simply very small bacteria but were a new type of tiny, disease-causing particle.