The innate immune system scans organisms for the presence of one of a handful of key identifying markers that are shared by viruses and bacteria. For instance, most bacteria have wiggly tails. Toll-like receptors scan for these. Or they look for a particular variety of large molecules—called lipopolysaccharides—that characterize a class of bacteria called gram-negative bacteria (such as E. coli); or they look for nucleic acids associated with viruses.
