RNA viruses

[Chris Barry]

Some viruses (M13) exist as a single strand of DNA while 
encapsulated in the protein coat. After they infect a bacterium, 
they inject the single strand and create a complementary strand to 
it. The complement (the replicative form when double stranded) 
creates more copies within the host while the other is packaged in a 
protein coat to infect other potential hosts. + and - just denotes 
which strand you are talking about.