T-SQL (SS2K8) :: Eliminating Divide By Zero Error

Being one step removed from innumerate, I was wondering whether there was a more elegant way to avoid divide by zero error instead of trudging through a bunch of isnulls.

My intuition tells me that since multiplication looks like repeated addition, that maybe division is repeated subtraction?
If that's true is there a way to finesse divide by zero errors by somehow reframing the statement as multiplication instead of division?

The sql statement that is eating my kishkas is

cast(1.0*(
(ISNULL(a.DNT,0)+ISNULL(a.rex,0)+ISNULL(a.med,0))-(ISNULL(b.dnt,0)+ISNULL(b.rex,0)+ISNULL(b.med,0))/
ISNULL(a.DNT,0)+ISNULL(a.rex,0)+ISNULL(a.med,0)) as decimal(10,4)) TotalLossRatio

Is there a way to nucleate the error by restating the division? My assertion underlying this statement is that the a alias represents a premium paid, so between medical, pharmacy and dental, there MUST BE at least one premium paid, otherwise you wouldn't be here. the b alias is losses, so likewise, between medical, pharmacy and dental, there MUST BE at least one loss (actually, it just occurred to me that maybe there are no losses, but that would be inconceivable, but ill check again)) so that's when it struck me that maybe there's a different way to ask the question that obviates the need to do it by division.