What Causes Miscarriage?

I am 40 years..n trying to get pregnant since 2011 October month...I got pregnant twice..firstly in august 2012 n October 2013 but both time got miscarriage within first trimester... had DNC n doctor suggested to torch test which came positive with rubella igg , Cylomegalovirus igg and herpes simple. Virus igg positive...is it curable?? Shall I try for next time after my course of medication suggested by doctor? I am very tense..plz guide me.