In your study, you cann't take the 0 month of the HIV infected people. Because, you did not know when they particular person was affected by HIV infection. So, normally we took the data called "''Censored Data and truncated data distribution" only.
So, you will take the data of 4, 6, 10 months period. Ok. Then only its correct. This is my opinion. Kindly refer the perivious journal papers related to your study.
It depends on the population you are studying (e.g. adults vs adolescents etc). I would suggest checking out the CAPS website (University of California, San Francisco) - they have a lot of survey instruments for HIV researchers, prevention planners etc - as long as you cite CAPS as your source.
There are many many questionnaires available but the most important thing is that what are your target groups? and in which cultural context do you want to go? overall there is five standard questions designed by UNAIDS and WHO to check correct information and knowledge of the participants regarding HIV transmission.But about the high risk behaviors it is very different based on the target groups
Can I suggest that you see what your own national blood bank questionnaire asks its blood donors -since these are a hetrosexual pool of donors. Many Tissue Banks also have a standard Risk assessment questionnaire to screen potential donors for risk behaviors. If you look up the American Assoication of Tissue Banks web site (aatb.org) you can see theirs- while an amount of these questions relate to the suitability of the tissue as well as the donor the risk behavior questions are good. The other places to ask are in the Genitourinary disease clinics- they would also have standard questionnaires.
Your problem is getting honest answers in some cases- how will you administer your questionniares - you need to have a non-judgemental open disclosure attitude so as not to bias the responses- even your written questions must make people comfortable to answer.