The taxonomy of this group is extremely difficult as the species have been placed in the genus Rhus as well as the genus Toxicodendron. There are also many subspecies and varieties, some of which seem to hybridize. The plants vary from low shrubs to climbing vines which helps to account for the fact that several species are referred to as both `poison-oak` and `poison-ivy.` An interesting fact is that both sensitivity and immunity vary with time as well as with individuals. In other words, someone who is immune today, may be susceptible tomorrow!