The colloquial termÂ moldÂ (orÂ mould; seeÂ spelling differences) is applied to a large and taxonomically diverse number of fungal species where their growth results in a moldy appearance of objects, especially food.Â The objects become discolored by a layer of fungal growth. Molds areÂ fungiÂ that grow in the form ofÂ multicellularÂ filaments calledÂ hyphae.Â Â A connected network of these tubular branching hyphae, called a mycelium, is considered a single organism. The hyphae are generally transparent, so the mycelium appears like very fine, fluffy white threads over the surface. Cross-walls (septa) may delimit connected compartments along the hypha, each containing one or multiple, genetically identicalÂ nuclei. The dusty texture of many molds is caused by profuse numbers of asexual sporesÂ conidiaÂ formed by differentiation at the ends of hyphae. The mode of formation and shape of these spores is traditionally used to classify the mold fungi.Many of these spores are colored, making the fungus much more obvious to the human eye at this stage in its life-cycle. In contrast, fungi that can adopt a single celled growth habit are calledÂ yeasts.
Molds are considered to beÂ microbesÂ and do not form a specificÂ taxonomicÂ orÂ phylogeneticÂ grouping, but can be found in the divisionsZygomycotaÂ andÂ Ascomycota. In the past, most molds were classified within theÂ Deuteromycota.Â Molds causeÂ biodegradationÂ of natural materials, that can be unwanted when it becomesÂ food spoilageÂ or damage to property. They also play important roles in biotechnology and food science in the production of various foods, beverages,Â antibiotics, pharmaceuticals andÂ enzymes. Some diseases of animals and humans can be caused by molds, usually as a result of allergic sensitivity to their spores or caused by toxic compounds produced as molds growÂ 
There are thousands of known species of molds which have diverse life-styles includingÂ saprotrophs,Â mesophiles, psychrophiles andthermophilesÂ and a very fewÂ opportunistic pathogens.Â They all require moisture for growth and there are some aquatic species. Like all fungi, molds derive energy not throughÂ photosynthesisÂ but from theÂ organicÂ matter on which they live, utilisingÂ heterotrophy. Typically, molds secrete hydrolyticÂ enzymes, from the hyphal tips. These enzymes degrade complexÂ biopolymersÂ such asÂ starch,Â celluloseÂ andligninÂ into simpler substances which can be absorbed by the hyphae. In this way molds play a major role in causingÂ decompositionÂ of organic material, enabling the recycling of nutrients throughoutÂ ecosystems. Many molds also synthesiseÂ mycotoxinsÂ andÂ siderophoreswhich, together with lytic enzymes, inhibit the growth of competingÂ microorganisms.
Molds reproduce through producing very large numbers of smallÂ spores,Â which may contain a singleÂ nucleusÂ or beÂ multinucleate. Mold spores can be asexual (the products ofÂ mitosis) or sexual (the products ofÂ meiosis); many species can produce both types. Mold spores may remain airborne indefinitely, may cling to clothing or fur or may be able to survive extremes of temperature and pressure.
Although molds grow on dead organic matter everywhere in nature, their presence is visible to the unaided eye only when moldÂ coloniesgrow. A mold colony does not consist of discrete organisms but of an interconnected network of hyphae called aÂ mycelium. All growth occurs at hyphal tips, with cytoplasm and organelles flowing forwards as the hyphae advance over or through new food sources. Nutrients are absorbed at the hyphal tip. In artificial environments such as buildings, humidity and temperature are often stable enough to foster the growth of mold colonies, commonly seen as a downy or furry coating growing on food or other surfaces.
Few molds can begin growing atÂ 4 Â°CÂ (39Â Â°F), the temperature within a typical refrigerator, or less. When conditions do not enable growth to take place, molds may remain alive in a dormant state depending on the species, within a large range of temperatures before they die. The many different mold species vary enormously in their tolerance to temperature and humidity extremes. Certain molds can survive harsh conditions such as the snow-covered soils of Antarctica, refrigeration, highly acidic solvents, anti-bacterial soap and even petroleum products such as jet fuel.
XerophilicÂ molds use the humidity in the air as their only water source; other molds need more moisture.
The KÅjiÂ (éº¹?)Â molds are a group ofÂ AspergillusÂ species, notablyÂ Aspergillus oryzae, and secondarilyÂ A. sojae, that have been cultured in eastern Asia for many centuries. They are used to ferment a soybean and wheat mixture to makeÂ soybean pasteÂ andÂ soy sauce.Â Kojimolds break down theÂ starchÂ in rice, barley, sweet potatoes, etc., a process calledÂ saccharification, in the production ofÂ sake,Â shÅchÅ«and other distilled spirits.Â KojiÂ molds are also used in the preparation ofÂ Katsuobushi.
Red rice yeastÂ is a product of the moldÂ Monascus purpureusÂ grown on rice, and is common in Asian diets. The yeast contains several compounds collectively known asmonacolins, which are known to inhibit cholesterol synthesis.Â A study has shown that red rice yeast used as a dietary supplement, combined with fish oil and healthy lifestyle changes, may help reduce "bad"Â cholesterolÂ as effectively as certain commercialÂ statinÂ drugs.
Other molds that have been used in food production include:
Alexander Fleming's famous discovery of the antibioticÂ penicillinÂ involved the moldÂ Penicillium, although the species identity is disputed (Penicillium notatum,Â Penicillium chrysogenumÂ orÂ Penicillium rubens).Â 
Howard Florey,Â Ernst Chain,Â Norman Heatley,Â Edward AbrahamÂ and teams of scientists in the UK and USA developed industrial-scale production of penicillin between 1941-45 and arguable started the use of antibiotics in medicine.Â Â 
Molds are ubiquitous in nature, and mold spores are a common component of household and workplace dust. However, when mold spores are present in large quantities, they can present a health hazard to humans, potentially causing allergic reactions and respiratory problems.
Some molds also produce mycotoxins that can pose serious health risks to humans and animals. Some studies claim that exposure to high levels of mycotoxins can lead to neurological problems and in some cases death. Prolonged exposure, e.g. daily home exposure, may be particularly harmful. Research on the health effects of mold has not been conclusive.Â The term "toxic mold" refers to molds that produce mycotoxins, such asÂ Stachybotrys chartarum, and not to all molds in general..These toxic properties may also be used to the benefit of humans Eg. penicillin from penicillium and so on. In low doses these toxins that could otherwise be deadly can be controlled to our benefit to fight of infection.
Mold in the home can usually be found in damp, dark or steamy areas e.g. bathroom or kitchen, cluttered storage areas, recently flooded areas, basement areas, plumbing spaces, areas with poor ventilation and outdoors in humid environments. Symptoms caused by mold allergy are watery, itchy eyes, a chronic cough, headaches or migraines, difficulty breathing, rashes, tiredness, sinus problems, nasal blockage and frequent sneezing.
Mold growth in buildings can lead to a variety of health problems. Various practices can be followed to mitigate mold issues in buildings, the most important of which is to reduce moisture levels that can facilitate mold growth.Â Removal of affected materials after the source of moisture has been reduced and/or eliminated may be necessary for remediation.