When people think about viruses, one of the first things that comes to mind is that they’re small. And to be honest, that's how I would describe them too. I mean, that’s how we discovered viruses in the first place: they passed through filters that trapped bacteria, so we knew this infectious substance had to be smaller than any known cell.

So when scientists came across the very first giant virus, they naturally assumed it must be a brand-new bacterium. In fact, this “mystery microbe” spent nearly a decade in a freezer before anyone realized it wasn’t a bacterium at all. 

This discovery opened the door to an entirely new world of giant viruses, which had gone unnoticed simply because no one was looking for something so large in the virus realm. And it also raised an important question that remains controversial: If some of these giant viruses cause disease in humans?

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Sources: 

B. La Scola, 2003: A giant virus in amoebae • C. Abergel, 2015: The rapidly expanding universe of giant viruses: Mimivirus, Pandoravirus, Pithovirus and Mollivirus • N. Brandes, 2019: Giant Viruses—Big Surprises • D. R. Wessner, 2010: Discovery of the Giant Mimivirus • B. La Scola, 2008: The virophage as a unique parasite of the giant mimivirus • F. Sakhaee, 2022: Detection of Mimivirus from respiratory samples in tuberculosis‑suspected patients • J. M. Claverie, 2018: Mimiviridae: An Expanding Family of Highly Diverse Large dsDNA Viruses Infecting a Wide Phylogenetic Range of Aquatic Eukaryotes • D. Raoult, 2006: Laboratory infection of a technician by mimivirus • N. Yutin, 2014: Origin of giant viruses from smaller DNA viruses not from a fourth domain of cellular life • M. Khan, 2007: Pneumonia in mice inoculated experimentally with Acanthamoeba polyphaga mimivirus • T. J. Rowbotham, 1983: Isolation of Legionella pneumophila from clinical specimens via amoebae, and the interaction of those and other isolates with amoebae • D. Raoult, 2007: The discovery and characterization of Mimivirus, the largest known virus and putative pneumonia agent • A. Levasseur, 2016: MIMIVIRE is a defence system in mimivirus that confers resistance to virophage • J. M. Claverie, 2016: CRISPR-Cas-like system in giant viruses: why MIMIVIRE is not likely to be an adaptive immune system • J. Abrahao, 2018: Lack of evidence of mimivirus replication in human PBMCs