There are tons of eucalyptus trees in the world. In Australia alone, the Eucalyptus genus makes up almost 75 percent of the total number of plant species! Some people call them "fever trees" because of their medicinal properties. Cough drops and other products used to help relieve chest congestion often contain eucalyptus oil. One species, the eucalyptus citriodora, is used more commonly in perfumes. This species has a lemon scent and mixes well with bergamot and lavender. Because eucalyptus oil is so potent, you don't need to use very much of it when creating a perfume -- making the oil a cost-effective and common ingredient.