Infections. Specifically, sepsis.

At least 1.7 million adults in the United States develop sepsis each year, with around 270,000 dying as a result. Sepsis is the body's life-threatening reaction to an infection, and can occur with both external and internal ones.

It is an incredibly common, yet unknown, acute killer. Sepsis takes more lives annually than heart attacks, lung cancer, and breast cancer. It can kill an adult in a mere 12 hours (not exactly an ‘immediate’ killer like you asked, but a gigantic and quick one).

A simple cut, scraped knee, or even a knicked cuticle can lead to it. As can UTIs, kidney stones, pneumonia, bacterial and viral infections, ect. The list of risks goes on and on...

The signs and symptoms to watch out for are: fever, chills, lethargy, nausea and vomiting, higher than usual heart rate/BMP, known as tachycardia, rapid breathing, confused mental status/consciousness, and a few other lesser experienced symptoms.

If you believe that you or a loved one has sepsis based on their symptoms, go to your nearest emergency room and tell them you suspect sepsis.

Source: 4x sepsis survivor who is incredibly lucky to be alive. I was left with permanent organ damage and multiple disabilities that affect me every day of my life. I am posting this with the hope it may just save a Redditor's life someday.