Can viruses kill other diseases ?

[Daniel Roger Bucknell]

I was just wondering, is there a virus that can for example infect a 
person with cancer, E-Coli or some other bacterial infection and then the 
virus kills off the offending bacteria if our immune system cannot ?  
Are there viruses that can do that at all so that instead of Hepatitis 
for example infecting and stuffing up our liver couldn't some virus be 
made to kill off other bacteria or even other viruses ?

Doh, not the best of wording there but you get the idea I hope, could you 
reply via email cause I can't use these newsgroups all that often.

Thanks, CYA