Lyme disease is classified as a zoonosis, as it is transmitted to humans from a natural reservoir among small mammals and birds by ticks that feed on both sets of hosts. Hard-bodied ticks of the genus "Ixodes" are the main vectors of Lyme disease (also the vector for "Babesia"). Most infections are caused by ticks in the nymphal stage, because they are very small and thus may feed for long periods of time undetected. Nymphal ticks are generally the size of a poppy seed and sometimes with a dark head and a translucent body. Or, the nymphal ticks can be darker. (The younger larval ticks are very rarely infected.) Although deer are the preferred hosts of adult deer ticks, and tick populations are much lower in the absence of deer, ticks generally do not acquire "Borrelia" from deer, instead they obtain them from infected small mammals such as the white-footed mouse, and occasionally birds. Areas where Lyme is common are expanding.