Cockroaches (or simply "roaches") are insects of the order Blattaria. This name derives from the Latin word for "cockroach", blatta. (insert hard return) There are about 4,000 species of cockroach, of which 30 species are associated with human habitations and about four species are well known as pests. (insert hard return) Among the best-known pest species are the American cockroach, Periplaneta americana, which is about 30 millimetres (1.2 in) long, the German cockroach, Blattella germanica, about 15 millimetres (1/2 in) long, the Asian cockroach, Blattella asahinai, also about 15 millimetres (1/2 in) in length, and the Oriental cockroach, Blatta orientalis, about 25 millimetres (1 in). Tropical cockroaches are often much bigger, and extinct cockroach relatives such as the Carboniferous Archimylacris and the Permian Apthoroblattina were several times as large as these. (insert hard return) Cockroaches are one of the most commonly noted household pest insects. They feed on human and pet food, and can leave an offensive odor. They can also passively transport microbes on their body surfaces including those that are potentially dangerous to humans, particularly in environments such as hospitals.Cockroaches infestations have been shown to be linked with allergic reactions in humans. One of the proteins that triggers allergic reactions has been identified as tropomyosin. These allergens have also been found to be linked with asthma.