What happens if the sperm donor is CMV positive?

CMV. As long as you are also CMV positive there should not be a problem. If you are negative, to play it completely safe, pick a CMV negative donor.
Probably nothing. CMV infects 70-80% of people and usually doesn't cause problems but in the fetus it can rarely cause mental retardation, deafness, blindness. Once exposed to cmv, the person's is protected from future infection due to formation of CMV antibodies by the immune system. Sperm from a CMV positive donor can potentially put the fetus of a cmv-negative recipient at risk, though this happens very rarely.