Eucalyptus are any of numerous tall trees of the genus Eucalyptus, native to Australia and having aromatic leaves that yield an oil used medicinally and wood valued as timber. There are over seven hundred varieties and many of them are known as gum trees due to the excretion of sap from any breaks in the bark. Humans have used Eucalyptus leaves and depend on Eucalyptus more often than they realize. Eucalyptus oil and its active ingredient eucalyptol are frequently found in the cough drops and salves you buy over the counter to treat colds and other infections, diseases and ailments. Eucalyptus has been used to lower the water table and reduce malaria and habitats for mosquito larvae to grow. Eucalyptus oils have been used for cleaning, deodorizing, and in very small quantities in food supplements like sweets, cough drops and decongestants. Eucalyptus oil Is also contains natural repellant properties and has been used in mosquito and flea repellants and also to kill dust mites. A couple other uses are in dyes and also, Eucalyptus is used to make the Australian Wind Instrument the Didgeridoo.

“It was also thought that the oils dropping from the eucalyptus leaves and the gums secreted from the bark, disinfected the ground around the tree. These secretions had a purifying effect just like its aroma did to the surrounding air. ”
“Abbott Kinney gave many examples of the success eucalyptus was having in arresting malarial fever”
“There were numerous reports worldwide of the success the eucalyptus was having in treating malaria.”
http://wwwlibrary.csustan.edu/bsantos/section2.htm#FIGHTING

Oils “Eucalyptus oil contains substances with strong antibacterial properties. Studies in animals and test tubes also found that eucalyptus oil acts as an expectorant (loosens phlegm in the respiratory passages), antiseptic (prevents infection), and deodorant. Like eucalyptus oil, the leaves of the eucalyptus plant contain substances that have expectorant, antibacterial, and antiseptic properties, but the leaves are also believed to help reduce inflammation and reduce fevers. In fact, one study conducted in Russia found that an alcoholic tincture containing eucalyptus leaves helps relieve chronic ear infections. Many researchers believe that the beneficial effect of the eucalyptus tincture may have been due to its anti-inflammatory properties.” Eucalyptus leaves contain tannins (which are believed to help reduce inflammation), flavonoids (such as quercetin which has antioxidants properties), and volatile oils
http://www.newleaf.com/common/adam/DisplayMonograph.asp?storeID=J3QSSEQX5CS92J2000AKHMCCQJA05T39&name=ConsHerbs_Eucalyptusch