Final answer:
Protozoa are single-celled, animal-like organisms that are found in decayed materials and contaminated water and can either be part of food webs or cause diseases.
Step-by-step explanation:
The one-celled animal-like organisms that are found in decayed materials and contaminated water are called protozoa. Protozoa are single-celled eukaryotes that have animal-like characteristics, such as the ability to move and heterotrophy, meaning they consume organic material rather than producing their own food. They include various forms such as flagellates, ciliates, and sporozoans. While some protozoa are beneficial in food webs, others can be harmful, like Giardia lamblia, which causes severe diarrhea when ingested.