Rubella was a mashup name of Ruby and Bela used by fans who were definitely not keen on the two new characters  introduced in Season 3. The fact that the word also meant a disease (aka German Measles) was appropriate for the level of vitriol that was expressed by some fans.
From the moment news of the new characters was leaked (see the casting sides below), many fans were concerned that the characters were going to be love interests for Sam and Dean, and detract from the brother's relationship.
We've always wanted to expand the universe and introduce new characters. And for us, it's about introducing them conservatively and in small doses. Because the fans are a protective and occasionally nervous bunch, we are always reiterating both in interviews and in practice that the show is about Sam and Dean. They are the two leads. It'll never be about anything else. It'll always be about the two of them, the issues they're going through, and then, first and foremost, their relationship with each other.
When we bring in Ruby and Bela, we don't bring them in where the whole episodes are about them. The episodes are about the boys. And the women come in, and they come in for a few scenes. They're there for important plot elements, but it's not the Ruby and Bela show, nor is it about the four of them cruising around in the Impala together. It's about the guys.