|“||The Others didn't invent death. They merely perfected it.||”|
The Waves were the name given to the coordinated attacks The Others used to try to wipe out humans on Earth. Cassie's father believed that The Others needed the Earth possibly because the conditions of their home planet were now uninhabitable. The truth is that The Others had learned of the Earth centuries ago and studied humanity in order to learn how to defeat it. The Waves were then thought of and they sent a fully automated ship to Earth to execute it though it took some time before it arrived.
In The Last Star, the 5th Wave is about to start, but due to Ringer and the sacrifice made by Cassie Sullivan, it was stopped.
The 1st Wave
- "Lights out."
The First Wave, as its name implies, was the first of the five (current) waves to be used by the Others. It was a massive EMP strike, an electromagnetic pulse, that effectively shorted out all forms of technology and objects that ran on electricity (i.e.: cars, cellphones, etc.). This had the effect of causing all cars and other motor vehicles to crash into each other or into buildings and brought airborne planes crashing from the sky.
Cassie mentioned that the first wave killed around half of a million people.
The 2nd Wave
- "Surf's up."
The Second Wave was a literal wave: judging from Cassie's recount, the Others dropped some form of a metal rod from the height of the atmosphere onto one of the earth's fault lines, which then resulted in earthquakes and massive tsunami waves that wiped out all coastal cities on all seven continents. This including several major metropolises such as New York, London, Paris, Tokyo, Sydney and Los Angeles. Cassie reasons this was because a great degree of the human population lived by the coasts.
This resulted in far more deaths than the first wave by a considerable amount. Cassie mentions the 2nd wave took out 3 billion people.
The 3rd Wave
The Third Wave starts as a plague that wiped out around 97% percent of the remaining human population. Also known as the "blood plague", "the Red Death", and "the Fourth Horseman", the Others genetically engineered this virus to have an almost-perfect kill rate. The 3rd Wave was spread throughout the globe by the use of birds as the carriers of the disease. With over 325 billion birds, "the Others couldn't invent a more efficient delivery system." The virus developed within the birds( via falling excrement)and then became airborne. People infected with the virus first exhibited cold-like symptoms, and it progressed from there to the point where "blood poured out of every inch of your body". Eventually, the virus would take hold of their victim's mind, and the infected person would die in a "series of seizures." The infected people turned into viral bombs that spread the infection to all the other people nearby. Lisa Sullivan (Cassie's mother) died from The 3rd Wave, Cassie described that Lisa "didn't recognize her," due to a total shutdown of the brain's nerves and response systems. There were some people, like Ben Parish, who were able to fight the virus off and recover.
The 4th Wave
The Fourth Wave was when the Others began to physically appear on Earth among the humans. They were called "Silencers", as it was their job to kill much of the remaining human population. They have programmed themselves into certain humans and have full control of them. This made it hard for the remaining human population to trust in anyone now as they cannot tell who is truly human or a silencer.
It is later seen that all the Silencers are really humans who were enhanced, most from the time they were fetuses in the womb, by use of alien nanotechnology. They were downloaded with false histories and memories of their times 'before being human' to convince them otherwise. The 12th System was developed to make more Silencers out of humans that they captured such as Ringer.
The Fourth Wave occurs in the first half of the scenario The 5th Wave takes place in, the other half being the titular Fifth Wave.
The 5th Wave"..."
The Fifth Wave is a group of highly trained young children and teenagers into finishing off whatever the Silencers didn't pick off from the fourth Wave—although these aforementioned "child soldiers" were fooled into thinking that they were assassinating the infested by Commander Vosch. Each of these kids were brainwashed and set in killing innocent people that glowed green due to the detector scopes given to them by the Others that would fool them into thinking they were killing Others instead of survivors. Only the members of Squad 53, Cassie, and Evan Walker know the truth.
The Fifth Wave was to begin when the spaceship launched missiles to destroy the cities on the planet in order to limit the places for people to be. Then the army would sweep them and hunt the survivors down. However, Ringer broke into Camp Haven II and activated the master control for the tracker's killswitches in all of the soldiers to zap them all.
When Cassie Sullivan blew up the spaceship and killed Commander Vosch, the invasion was thwarted (along with the fact that the majority of the child soldiers were stopped by Ringer). The child soldiers leftover from the 5th Wave departed from the military bases and it is unknown if, at the end of The Last Star, are still hunting the "infested." While with Sam and Megan, Ben Parish assumed that most of them would just be looking to survive. One was spotted by Ben and Sam in an abandoned store in full gear but had shot himself.
Silencer remnants of the 4th Wave also remained and, in a conversation with Ben Parish, Evan Walker theorized that without the ship they might try to regroup at military bases to access any of the remaining technology or weapons left by the Others. At the finale of The Last Star, Evan Walker embarked on a solo mission to destroy all military bases previously controlled by the Others in North and South America to prevent more innocent people from dying.