Hey, future environmentalist! We're about to dive into the world of diseases, pathogens, and how they change the game for all living organisms. Let's get the party started!

Disease is like that uninvited guest who crashes your party and ruins all the fun. It's an illness or infection, usually caused by a pathogen. It's like having a friend who always borrows your clothes and never gives them back - pretty harmful, huh?

Pathogens are the "bad guys" in this story. They're harmful organisms, which can be bacteria, viruses, fungi, or some single-celled organisms. Think of them like the villains in a superhero movie. One example is Plasmodium, a single-celled protoctistan who is famous (or should we say infamous?) for causing malaria.

Real-World Example: You remember that movie where the villain tries to take over the world? Pathogens are kind of like that. They sneak in and start causing chaos. A real-world example would be the COVID-19 virus. It's a pathogen that has had a major impact on our world.

Now, let's dive into some fancy terms. A pathogen reduces the 'carrying capacity' of the organism it's infecting. If you imagine a bus full of people (your body), the pathogen is like a passenger who takes up five seats. Suddenly, there's less space for everyone else!