An attack by biological agents could result in the release of deadly toxins, viruses, germs, bacteria or disease that can kill everything in its path, and having a solid emergency plan created in advance may be the only thing that prevents you and your family from becoming one of the many statistical fatalities.
Aside from nuclear meltdowns occurring worldwide post TEOTWAWKI, deadly diseases and viruses spreading worldwide is arguably the next biggest danger to humanity. History is wrought with cases of deadly pandemics killing off in some cases up to 50% of a regions population. Governments are currently doing a great job at quarantining outbreaks but every so often a virus becomes pandemic and uncontrollable. If for whatever reason the system responsible for vaccinations/quarantining and viral research falls apart, the population worldwide (especially in large cities) would quickly succumb to deadly pandemics. Fortunately, there are things you can do right now to prepare for such a crisis.