There are many reasons to examine human cells and tissues under the microscope. Medical and biological research is under-pinned by knowledge of the normal structure and function of cells and tissues and the organs and structures that they make up. In the normal healthy state the cells and other tissue elements are arranged in regular recognizable patterns. Changes induced by a wide range of chemical and physical influences are reflected by alterations in structure at a microscopic level and many diseases are characterized by typical structural and chemical abnormalities that differ from the normal state. Identifying these changes and linking them to particular diseases is the basis of histopathology and cytopathology, important specializations of modern medicine. Microscopy plays an important part in haematology (the study of blood), microbiology (the study of microorganisms including parasites and viruses), and more broadly in the areas of biology, zoology and botany. In all these disciplines specimens are examined under a microscope.