Most fungi are large enough to be seen with the eye. However, some are microscopic organisms and the study of microscopic fungi is encompassed by the field of microbiology. Some microscopic fungi, for example yeast, are used in the food and drink industry to produce bread, beer and wine. Other fungi are important in the pharmaceutical and biotechnology industries and are used in the production of antibiotics and various enzymes.
Some fungi are being used as probiotics. Saccharomyces boulardii, a species of yeast has been described as a biotherapeutic agent and the yeast has been used in clinical trials for the prevention and the treatment of intestinal infections and inflammatory bowel disease. Curr. Issues Mol. Biol. (2009) 11: 47-58
Some species of fungi are pathogenic to animals (including man) and plants. Medically important pathogenic fungi include Candida albicans, Aspergillus fumigatus and Cryptococcus neoformans.
Bacteria, viruses, amoebae and microscopic fungi are classified as microorganisms. The study of microorganisms is called microbiology.
Recommended reading at Fungi and current topics on microbiology research at the Microbiology Blog