Final answer:
Thrush (Candidiasis) is primarily caused by a fungus, specifically Candida albicans, which is a normal part of the human microbiota but can overgrow and become pathogenic under certain conditions.
Step-by-step explanation:
Thrush, also known as Candidiasis, is primarily caused by a type of microorganism known as a fungus. The specific fungus responsible for thrush is Candida albicans, which is a yeast cell that can become pathogenic when the body's normal microbiota is disrupted. Conditions such as antibiotic use, diabetes, and immunodeficiencies can lead to an overgrowth of Candida, resulting in oral thrush, vaginal yeast infections, or cutaneous candidiasis. Anti-fungal drugs are the common treatment for such infections, with targets more limited than antibiotics or anti-viral medications due to the similarities between fungal cells and human cells.