While not a single organism, lichens represent a symbiotic relationship between a fungus and an alga or a cyanobacterium. They are crucial for their role in soil formation and as a food source for various animals in arid environments.

These are infectious agents composed entirely of protein. Prions cause degenerative brain diseases in animals and humans, such as mad cow disease (BSE) in cattle and Creutzfeldt-Jakob disease in humans. They are unique among pathogens as they lack nucleic acids (DNA or RNA).

Including yeasts, molds, and mushrooms, fungi are mostly multicellular (except for yeasts, which are unicellular). They play crucial roles in decomposition and nutrient cycling and are also used in food production (like bread and cheese making) and medicine (antibiotics like penicillin).

Single-celled organisms that can be found in almost every environment on Earth. They come in various shapes like rods, spirals, and spheres. Some bacteria are beneficial (like those in our gut flora), while others can cause diseases.