What infections are particularly dangerous in pregnancy?

Many. Hiv, cmv, rubella measles, parvovirus or fifths disease, syphillis, chicken pox, and even swine flu, to name a few.
Depends on to whom. To the fetus, rubella, toxoplasmosis, cytomegalovirus, listeria in particular. To the mother, and indirectly to the fetus, influenza, chickenpox, coccidiodomycosis and hepatitis e as examples.