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Viruses could pass through bacterial filter which made belief that viruses are fluid.

Contagium vivum fluidum is the term that was used by the scientists to describe the virus. The virus was first discovered by the scientist Beijernick who isolated the tobacco mosaic virus and as the tried to filter and separate the virus through the finest of the filters possible, he couldn't. So he said that virus are fluid in nature. Later Ivanovsky also gave the same postulate. He crystallized the virus too.

But only after the invention of electron microscope, the structure and composition of virus were completely known.