Transmission of disease from mother to fetus
Pregnant mothers can indeed pass on various microbes to their fetuses but it is not always directly through the placenta, as the placenta can be protective.  Although the blood of the mother and fetus do not mix directly, the two can interact.  Maternal proteins can flow across to the fetus, as maternal antibodies are actually the source of a newborn's immune system for a few months, and maternal immune cells have even been found in fetuses, which of course means other cells or viruses can get across.

HIV is a good example for your question, actually.  HIV can infect newborns through the placenta, but the majority of infections occur during birth, not in utero.  Birth is a pretty bloody process, and the rupturing of the amniotic sac exposes the infant to anything in the mother's blood, which is when most infections occur.  For HIV specifically, physicians can all but prevent mother-to-infant transmission by administering anti-HIV drugs just before birth.

Here is a list from the CDC of some STDs which details how infections may be passed on to newborns; most clarify that it mainly happens during the birthing process.  That being said, plenty of diseases will cause transplacental infections.  Usually they are viruses, as viruses are simply smaller, but there are definitely bacterial and parasitic examples, such as Borrelia duttonii, Trypanosoma cruzi, and Toxoplasma gondii.  Here's a list of bacterial ones and here's a list of viral ones.