http://www.answersingenesis.org/docs/1346.asp
Diseases on the Ark
Answering the Critics
by Carl Wieland
"Some diseases afflicting humans today, whether caused by bacteria, viruses or parasites, are transmitted only from human to human, and the disease-causing organisms (pathogens) seem to be unable to survive for any significant time outside of an infected human host.
Would this not mean that members of Noah's family had to be infected with tapeworm, AIDS, syphilis, smallpox, measles and many more? This is a common 'problem' raised by anti-creationists in order to lampoon the biblical account of Noah and his family on the Ark during the Flood..."
Possibilities:
1) Specialization of the Pathogen
By this means, some disease-causing organisms may have been much less particular about their chosen host, and could thus have come through the Flood in some of the tens of thousands of animal species carried on board the Ark (just as today, tuberculosis carried in cattle can infect human beings), only later 'devolving' (specializing) into their present 'human-only' status. (See also the Appendix.) Alternatively, some which now only survive inside a human body may have been robust enough to survive outside of any host.
A virus is nothing much more than a protein coat and a single packet of information (RNA or DNA). It has no complex cellular machinery,
and should not really be called 'alive'. It hijacks the machinery of an existing cell. In computer language, it is really analogous to a piece of 'software' which modifies the software of a living cell so that the 'hardware' of that cell can make copies of the virus 'software'. It is a program for making copies of itself using machinery it does not possess. Since in evolutionary theory, fully fledged cells had to exist before viruses, the latter are not some evolutionary intermediate between life and non-life. Mutational shifts in viruses are not on the way up to a 'higher' form of life â€â€
a
virus has never been observed to give rise to anything other than a virus. No informed evolutionist should use mutational change in viruses as a defence of molecules-to-man evolution.
For full article:
http://www.answersingenesis.org/docs/1346.asp