Best Answer: Pros: They help in the decaying process. All things that die must decay. Otherwise this world would be full of dead bodies. Viruses, bacteria, fungi, algea and protozoa play the most important role in this process. Cons: Viruses, bacteria, fungi, algea and protozoa cause many diseases.Source(s): RN. SARJEET · 6 years ago.therwise this world would be full of dead bodies. Viruses, bacteria, fungi, algea and protozoa play the most important role in this process. Cons: Viruses, bacteria, fungi, algea and protozoa cause many diseases. Source(s): RN.