During the first wave the power went off because . a massive EMP strike, and an electromagnetic pulse happened.
The second wave was fatal, the Others(The bad people )dropped some form of a metal rod from the height of the atmosphere onto one of the earth's fault lines, that made happen earthquakes and tsunami waves that almost destroyed all seven continents.
During the third wave a plague spread and killed around 97% of population. This disease was called the "Red death."The 3rd Wave was spread throughout the globe by the use of birds as the carriers of the disease. This helped the other to get a good ratio kill.