One of the biggest problems is that so much about this virus (bird flu) and how to deal with it is unknown and because this fact, there is no real established immunity (resistance) known.
Things we do know about the bird flu are, this virus is very aggressive and multiplies rapidly and over half of those humans who have become infected have died. Proving that currently there is no vaccine/drugs that may be effective for preventing or really treating the bird flu.
Tamiflu has shown to boost the survival rate of mice infected with the H5N1 strain of bird flu but the virus is already becoming resistant to Tamiflu and nearly seven percent of people who are prescribed Tamiflu are not able to tolerate the side effect.