First addressing people complaining about the cheesy acting and odd humor in this movie. I've watched his movies many times and he likes to include the 1950s, early 60s sci-fi Bmovies deadpan, corny acting/humor in his movies. Watch Signs and really hear some of the things the actors say. Hear and observe how they say the lines. Its his signature lines and macabre humor that are uniquely him! The Happening has the same thing. If a viewer isn't familiar with the corny 50s movies, they won't get M Night Shyamalan and his referred humor. This humor cuts some of the gruesome scenes and lightens the heaviness.
The concept of nature fighting back or in essence defending itself is unique and taps into reality in many senses. The pandemic references are there. This movie also takes the macabre actions of people who are thrust into a rather panic situation quickly. It's a good storyteller that can take a oddball event and make it scary. These people couldn't run anywhere from the growing threat because it became very evident that anywhere was dangerous. Yes, the danger eventually subsided, things appeared to return to "normal" but just as anything else, a lull doesn't mean safety lasts because all too soon, the balance is upset and the assault returns. Nature seeks balance, that comes by eliminating the perpetrator, mankind. M Night Shyamalan has the best endings to his movies because it's never "all well that ends well". There's always that threat on the horizon.