The required test is focussed almost entirely on tuberculosis, Syphilis, alcoholism (I wonder how they test that...) and obvious (open) diseases. There is a lung radiography and blood samples are taken.
As far as I know, there is no gene test (which would serve, given the high amount of incest marriages, a definite reason) and no specific HIV/HSV/HPV test taken.

If a person passes the test, a medical certificate is issued and the civil servant (notary) is only allowed to perform the marriage, when both partners can present such a certificate.
If a person does not pass the test and further medical attention is needed, it can be confined to an hospital.

Please understand that this medical test has a certain reason: There are no serial or compulsory medical tests in Tunisia, like in most European countries. It can, therefore, happen, that persons almost never see a doctor in their life unless they are seriously ill. In effect, Tunisia has almost no epidemiological data. But there is one way to obtain it: Almost all persons in Tunisia marry, and usually they do it when they are young. So, at this point in time, they require a medical test and, as a result, obtain the required data of the public health of almost all young adults.

However, in reality, quite a few tunisians will not really being tested, but rather buy the certificate for a lump sum from a doctor. The result of this is then, of course, an increased official state of public health (ill persons will more likely buy the certificate) - and the fact, that you cannot rely on this medical certificate.