Ancient DNA in ice, such as the Ötzi the Iceman (Handt "et al." 1994), or through rapid desiccation, such as high-altitude mummies from the Andes (c.f. Pääbo 1986; Montiel "et al." 2001), as well as various sources of artificially preserved tissue (such as the chemically treated mummies of ancient Egypt). However, mummified remains are a limited resource. The majority of human aDNA studies have focused on extracting DNA from two sources that are much more common in the archaeological record – bone and teeth. Several other sources have also yielded DNA, including paleofaeces (Poinar "et al." 2001) and hair (Baker "et al."
Ancient DNA 2001, Gilbert "et al." 2004). Contamination remains a major problem when working on ancient human material. Ancient pathogen DNA has been successfully retrieved from samples dating to more than 5,000 years old in humans and as long as 17,000 years ago in other species. In addition to the usual sources of mummified tissue, bones and teeth, such studies have also examined a range of other tissue samples, including calcified pleura (Donoghue "et al." 1998), tissue embedded in paraffin, and formalin-fixed tissue. Efficient computational tools have been developed for pathogen and microorganism aDNA analyses in a small (QIIME) and large scale