Viruses are generally considered to be amongst the smallest bioactive particles; dating back to the original observations, including those of luminaries such as Ivanosky and Beijerinck, size has always been at issue within the definition, a tradition that continued for many years. It was thus a surprise to the scientific community in the early 2000s when French scientists demonstrated that a particle, previously thought to be a bacterium, was indeed a virus. The discovery of the Mimivirus and the other “giants” that have followed, including Mamavirus, Pandoravirus, Faustovirus, and Mollivirus, has blurred the definition of what constitutes a virus and, indeed, the boundaries between viral particles and cellular life.

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