Eucalyptus essential oil, from the eucalyptus tree originally native to Australia, has been used for centuries to treat chest and other respiratory problems. It is a very versatile oil that is anti-inflammatory, antiseptic, antibiotic, diuretic, analgesic, and deodorizing. Research has proven eucalyptus has antiviral properties as well.
You probably know about its effectiveness against coughs and colds because several over the counter cold medications list eucalyptus as an ingredient, including many cough drops. It is similarly effective in the treatment of cystitis and sunburn and can be used as an insect repellent for humans and pets. I also sometimes use a few drops of eucalyptus in my CPAP machine.