In DNA it is called "sense" and "antisense" - this is just which way the DNA twists - like which way the thumb sticks out of your hand is based on which hand it is... the DNA in a nucleus that is "read" (transcribed) into mRNA is "antisense" and for this reason the mRNA that results (when the DNA is translated) is called "sense" -because it is like a backwards imprint... in viri (the "correct" Latin/scientific for "viruses") positive sense means that the viral RNA looks to the cell just like mRNA coming from the nucleus... so it can be *translated* (by the ribosome) into amino acid sequences used to make a protein... when a virus infects a cell, it will use one type of RNA or the other (if it is an RNA virus anyway) to get the cellular machinery (the ribosome) to make proteins that help the virus, and often harm the cell... if the virus releases negative sense RNA it has to be copied (and thus reversed) into positive sense RNA by a different cellular "machine" called RNA polymerase before the ribosome can read and thus translate it into aa sequence.