The First "Virus Hunters".

The history of virology is a history of conceptual and technological inventions and breakthroughs. The development of filters made of porcelain or kieselgur by the end of the 19th century which withheld bacteria allowed the identification of infectious agents smaller than bacteria and noncultivable on the media known at that time and used to grow bacteria. Even finer-grain filters resulted in the observation that the ultravisible novel infectious agents are in fact of particulate nature. Infections of plants and animals were the first to be attributed to these tiny entities. Proof resulted from experimental infection of the natural hosts (including humans). Thus, of the first 30 viruses identified, 20 are veterinary viruses, i.e. infectious agents of poultry and livestock. The discovery that bacteria also have viruses in the 1910s expanded the viral universe which continues today. Filterability and ultravisibility remained a hallmark for the identification of viruses until the advent of the electron microscope in the late 1930s marking another technological breakthrough in virology. Cell culture techniques allowed virus propagation outside the infected organism. In the past decades, the advent and development of molecular biology has brought more innovations culminating in the rapid and accurate determination of genomic material of a variety of living beings including viruses in a hitherto unknown speed and depth using next-generation sequencing and metagenomic analyses. Thus, it is no surprise that new viruses are detected constantly including specimens of unprecedented size and shape. Virologists agree that the viral universe is immense, and only a small fraction has been explored yet.