JIM DOWDALLS / SCIENCE PHOTO LIBRARY JIM DOWDALLS / SCIENCE PHOTO LIBRARY
This illustration depicts a number of smallpox viruses, one of the most complex type of animal viruses known. The structure of the smallpox virus contains one strand of double-stranded DNA contained in a rounded bioconcave brick-like core (purple). This core is surrounded by several layers of host cell membranes (aqua), which in turn is surrounded by an outer protein coat (yellow).
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