Yeasts are best described as unicellular fungi because they are eukaryotic organisms that reproduce mainly by budding or fission, unlike multicellular molds. They have a rigid cell wall made of chitin and glucans, characteristic of fungi, and lack the structures and metabolic features of bacteria or viruses. Clinically, yeasts like Candida species are important pathogens causing infections, especially in immunocompromised patients. Identifying yeasts correctly aids in selecting appropriate antifungal treatment, distinguishing them from bacterial or viral infections.

Reference: Medical Microbiology, Murray et al., 9th Edition.