Microbes
Microbes are tiny organisms that are too small to be seen from naked eye (need the help of microscopes to be seen).
There are 3 main types of microbes:
Microbes live in a wide range of habitats on earth, even some of the most extreme environments (extremely hot, acidic...) However, all microbes have ideal conditions to grow and reproduce (e.g. pathogens tends to grow at 35 ~ 37 °C, the temperature similar to human body).