Viagra is definitely big. The last two (and the only two times) my WordPress blog has been hacked it was loaded with hidden pharmaceutical links. I think people can probably earn well with pharmaceutical affiliate programs but let me tell you what I think they really do or at least the majority of them. I think the majority of the viagra spam originates from criminals that buy fake pills from China for 1 cent a piece and then sell them on at ~$4-$8 per pill via spam, hacking and everything else the criminal arsenal has to offer.