mocktwinkie saidIt's the general principle that people could be just walking in with communicable diseases and there's no way to find out where they may be coming from. The third world has numerous diseases.
Where do you think HIV came from? ...Haiti, most likely.
A few moments thought will demonstrate the stupidity of your proposal.
Firstly, when HIV was originally brought into the US, it was not understood what caused the disease was nor how to detect it.
Secondly, disease is not only communicated by humans; it is more often carried by vermin, etc. As much as we might wish to screen the rodents entering the country (whether from land borders or shipping), the task is manifestly futile.
Thirdly, we should ask ourselves the following question: are illegal immigrants a significant or leading cause of the introduction of disease? It seems highly unlikely that this is the case. Firstly, because illegal immigrants are likely to cross the border significantly less frequently than the average citizen (for obvious reasons) and secondly because epidemic transmission in its initial stages proceeds through small numbers of highly improbably events; we should look to those who travel legally and frequently to exotic places, and legal visitors to the US from such climes for the the most likely mode of introduction.