Muffins,

All you need is multiple labs or global tech partners/sub labs. All running the same security protocols in the containment areas. Let's just say the same contractor was awarded the contract for that Bio Tech firm. So, everywhere you have a lab, you have that same contractor responisble for all non physical security measures..
You get a hacker or group of hackers to release a virus and it opens all the rooms the patients are in.. they run amok all at the same time.
A biological attack can be pulled off simultaneously and transnationally, but the logistics to do so, are damn near impossible.
An electronic signal is the most probably way because distance has no effect on it.
Flight attendents aren't going to reach each country at the same time due to distance and flight times. Germany is an 8 hour flight from NYC, Italy is around 10 hours, reaching Asia from the LA is around 11 hours and reaching the middle east from NYC is even longer.. At the rate they turn, the planes would be "landing" with everyone dead or undead except the pilots.