They might be saving 9 million lives around the world every year, but vaccines can't seem to escape the criticism by a vocal minority that they contain many different kinds of harmful ingredients. But as Henry explains in the latest episode of MinuteEarth above, that's exactly the point - vaccines wouldn't even exist if it weren't for these dangerous ingredients, and the only reason you should be scared of them is if you don't understand how vaccines actually function. 

On their own, our bodies are actually really great at warding off disease-carrying pathogens. Once the pathogens get inside, our immune cells will kick into gear, engulf a few, and send out a chemical alarm based on the molecular structure of the captured pathogens. This alarm triggers antibodies and specialised immune cells to seek out the rest of the pathogens and take them down.

Once the bloody battle has been won, a few of these specialised immune cells - Henry calls them assassin cells - will stick around, in case the same type of pathogen makes its way into our system again. This time around, the body's immune response will be even faster, because it's mounted this type of attack once before.

But what about those incredibly advanced diseases that have evolved to disable our initial immune response before it can raise the chemical alarm? That's where vaccines come into play. Containing a weak dose of dangerous pathogens, vaccines allow our immune system to perform a 'practice run', says Henry. Or they might contain harmful chemicals such as formaldehyde, because in very small amounts, this chemical can alter the structure of pathogens responsible for diseases such as tetanus and influenza, and render them safe.

Still worried about the formaldehyde in your vaccines? The only thing to really be concerned about is if the chemical is in such small amounts, it renders the pathogens so useless, they don't even trigger an immune response. That's where a tricky little ingredient called an adjuvant comes into play. Whats an adjuvant, I hear you ask? I'll let Henry explain in the episode of MinuteEarth above, but let's just say that those fears you might have about vaccine ingredients - no need to worry, scientists have got you covered.