Fungi are a taxonomic kingdom of eukaryotic organisms, the most familiar of which are mushrooms, yeast, mold, and mildew. Thousands of different types of fungi exist on Earth. They can be found in air, in soil, on plants, and in water. Some live in the human body, usually without causing illness.
Containing over 100,000 species, Fungi are classified according to their method and structures associated with reproduction.(Miller, p 530) Once mistaken as plants, fungi were proven to be completely different and placed in a separate kingdom. One distinguishing characteristic of fungi is the presence of chitin (a complex carbohydrate) in the cell walls. Fungi also do not perform photosynthesis, but rather are heterotrophs, depending on other sources for their food.(Miller, p 527) These unique organisms are essential in almost every ecosystem because of their ability to decompose and recycle nutrients. Other fungi however, are harmful parasites that cause multiple diseases and infections.(Miller, p 538)
All fungi are multicellular with the exception of yeast.(Miller, p 527) Their cells are made up of thin, stringy filaments known as hyphae. Hyphae have many nuclei, and some have cross walls called septa that separate each cell. A mass of hyphae is the main body of the fungus, known as the mycelium.  The mycelium is well adapted for taking many nutrients because it has a large surface area.(Miller, p 528) The mycelium is usually underground, hidden in a food source or soil. 
The cell walls of fungi are composed of a carbohydrate found in the exoskeletons of arthropods and insects (called chitin). The chitin provides support and structure for the hyphae cells. Reproductive hyphae form a sporocarp or mushroom for spore distribution. . Two other types of hyphae are stolons and rhizoids. Stolons run along the surface of an object. Rhizoids are like roots that work into a food source or object, breaking down and absorbing nutrients.(Miller, p 530)
Fungi can reproduce both asexually and sexually. Asexual reproduction occurs when specialized hyphae called sporangiophores release haploid spores made in the sporangia. Fungi can also break off cells or hyphae that then grow into new organisms.(Miller, p 528) Yeast grows asexually through a process known as budding.(Miller, p 533) Sexual reproduction varies among different phyla of fungi, but all follow a basic pattern. Two different mating types, known as minus (-) and plus (+), fuse together and create a cell containing two nuclei (one for each type). After a period of time, the nuclei merge and form a diploid zygote. This zygote then goes through meiosis and produces haploid spores that can be released to grow on their own.(Miller, p 529)
This sexual process occurs using a fruiting body that grows from the underground mycelium. This fruiting body is usually recognized as a mushroom, and mushrooms growing close together are commonly from the same fungus. "Fairy rings" can be formed when one mycelium lives for many years and depletes the soil in its center. This causes the fungus to grow fruiting bodies only on the outer rim of the mycelium, producing a ring.(Miller, p 528) A single mushroom can produce billions of haploid spores, while a large puffball can produce trillions of spores.(Miller, p 535)
Each phylum is categorized by its unique method of reproduction. Zygomycetes for example, use a zygospore ("a resting spore that contains zygotes formed during the sexual phase of the mold's life cycle") during sexual reproduction.(Miller, p 530) Ascomycetes have a structure called an ascus that is used for sexual reproduction, as well as conidia (tiny spores) for asexual growth.(Miller, p 532) Club fungi, or basidiomycetes, have a club-like structure containing spores known as basidia. These basidia are located along the gills of mushrooms.(Miller, p 534)
Fungi are heterotrophic, which means that they don't make their own food like plants. Instead, fungi must obtain nutrients and energy from other organisms. To do this, they release digestive enzymes that digest the food outside of the mycelium. Digested food is then absorbed by the fungus. Most fungi are saprophytes, organisms that feed on decaying or dead organic matter.  For this reason fungi are vital to almost every ecosystem because they recycle minerals and nutrients. This keeps the soil from becoming depleted and worn out. Some fungi are parasites and live off of their hosts. Parasitic fungi cause diseases in plants, animals, and even humans. Plant diseases include corn smut, wheat rust, and mildew. The genus Cordyceps is made up of different fungi that infect animals, including one that eats grasshoppers alive. Common human diseases due to fungi are athlete's foot, ringworm, thrush, and yeast infections.(Miller, p 538-39)
A few types of fungi however, develop symbiotic and beneficial relationships with other organisms. One example of this is mycorrhizae, an association between plants and certain fungi. Mycorrhizae provides a larger surface area for the plant roots and obtains extra water, nutrients, and minerals for the plant. In exchange, the plant gives the fungus the energy and sugars produced through photosynthesis. 
In fact, some plants are so dependent on their mycorrhizal fungi that they can't survive without them. Orchids, for example, are unable to germinate without their partner fungi. It is estimated that 80% of all plants form beneficial mycorrhizae with fungi. Some of these relationships are very specialized, such as the fly agaric that grows with birch and pine trees. Underground, there are entire networks and systems of mycorrhizae that connect numerous plants. This enables nutrients and materials to be sent from one plant to another, increasing the growth of the various plants.(Miller, p 541-42)
Lichens are also mutualistic associations, but between a fungus and a photosynthetic organism. This organism is usually a cyanobacterium or a green alga, and the lichens collect water and food for these organisms and protect them from the sunlight. In return, they receive photosynthetic energy. Lichens can grow almost anywhere and survive very harsh conditions. Unfortunately, they are very delicate when it comes to air quality, and die easily of air pollution.(Miller, p 540)
Fungi are also very important to normal human activities. For example, yeasts are used to make breads, beer, and wine. Vats of fungi are grown to create enzymes to remove stains, produce flavorings for cooking, and even vitamins. Many edible mushrooms are favorite delicacies, and some are extremely rare. Morels and truffles are two valuable and delicious mushrooms that are hunted because of their remarkable taste. In fact, truffles in particuluar are so difficult to find that a pound of them can cost hundreds of dollars. Some fungi, such as Penicillium notatum, are used to make antibiotics. The drug penicillin kills off bacteria and germs, saving many people from death due to bacterial diseases.  Other fungi, especially yeasts, are ideal model organisms for molecular biology and genetics. 
In China, a parasitic fungus that attacks caterpillars has been used as a medicine for centuries. It is used as a strength restorative and tonic. Controlling moth populations that harm farmland is also a possible use for this fungus.  Some types of fungi are also used to make drugs that keep the body from rejecting organ transplants.  Fungi are used as biocontrols, replacing expensive and harmful pesticides. Biocontrol utilizes a specific organism to kill off another one. The fungi that are used are usually parasitic, especially for insects such as ants, spittlebugs, and potato beetles. Some fungi are even used to control other fungi that harm plant roots or cause disease. 
- Main Article: Infectious disease
Fungi can cause of number of diseases which are collectively called mycoses. Mycoses can affect your skin, nails, body hair, internal organs such as your lungs, and body systems such as your nervous system. Aspergillus fumigatus, for example, can cause aspergillosis, a fungal infection in your respiratory system.
Fungi can cause diseases of plants as well. Scientists who work with wheat are concerned about new kinds of a fungus called rust, which attacks wheat plants. The fungi from the genus Puccinia, called stem rust and yellow rust, have started to even infect wheat that has a genetic resistance to these fungi. Biologists are working to find strains of wheat that will resist these new wheat pathogens.
Striate bird-nest fungus (Cyathus striatus)
Pigskin Poison Puffball (Scleroderma citrinum)
Honey Fungus (Armilliara mellea)
Barometer Earthstar (Astraeus hygrometricus)
Agaric Mushroom (Amanita muscaria)
Scarlet Elf Cup (Sarcoscypha coccinea)
Devil's Urn (Urnula craterium)
Candlesnuff Fungus (Xylaria hypoxylon)
Sporangia of mucor mold
- Introduction of the Fungi by Ben Waggoner. UCMP (University of California and Museum of Paleontology).
- Fungi: More on Morphology by Ben Waggoner. UCMP (University of California and Museum of Paleontology).
- Fungi- The Hidden Kingdom by Robert Fogel. Fun Facts About Fungi.
- Penicillin: The First Miracle Drug by Robert Fogel. Fun Facts About Fungi.
- Caterpillar Fungus: A Traditional Medicine by Lai Ming (Vivian) Luk. Fun Facts About Fungi.
- Fungal Terminators: Biocontrol by Robert Fogel. Fun Facts About Fungi.
- Fungi by Meredith Blackwell et al. Tree of Life Web Project.
- Miller, Levine. Prentice Hall Biology. 2008. Chapter 21: Fungi (pg 526-547).
- Rust Never Sleeps A new flare-up in an age-old battle between wheat and a fungal killer, By Rachel Ehrenberg, Science News, September 25th, 2010; Vol.178 #7 (p. 22). Accessed September 15, 2010.