Eucalyptus was brought to California from Australia to drain swamps; now, it’s everywhere. Despite its invasive ecology, we still love it, not least for the heady smell it imparts to our morning walks. That scent changes a little once the leaves are steam distilled into essential oil—that’s because distillation only captures molecules of a certain size. Medicinally, though, those are the ones that count: Eucalyptol (1, 8-cineole), the active ingredient of the eucalyptus oil, is responsible for its various pharmacological actions. Eucalyptus oil is super antimicrobial, and has exhibited antibacterial activity against Gram-positive as well as Gram-negative bacteria—including those resistant to commonly used antimicrobial agents.  It’s also has analgesic (pain-relieving) and anti-inflammatory properties, making it great for sore muscles. 
Find it in
 Trivedi, N. A., and S. C. Hotchandani. "A study of the antimicrobial activity of oil of Eucalyptus." Indian Journal of pharmacology 36.2 (2004): 93.
 E. R. Hendry, T. Worthington, B. R. Conway, P. A. Lambert; Antimicrobial efficacy of eucalyptus oil and 1,8-cineole alone and in combination with chlorhexidine digluconate against microorganisms grown in planktonic and biofilm cultures. J Antimicrob Chemother 2009; 64 (6): 1219-1225. doi: 10.1093/jac/dkp362
 Trivedi and Hotchandani.
 Sherry, Eugene, Harry Boeck, and Patrick H. Warnke. "Topical application of a new formulation of eucalyptus oil phytochemical clears methicillin-resistant Staphylococcus aureus infection." American journal of infection control 29.5 (2001): 346.
 Göbel, H., G. Schmidt, and D. Soyka. "Effect of peppermint and eucalyptus oil preparations on neurophysiological and experimental algesimetric headache parameters." Cephalalgia 14.3 (1994): 228-234. [Link]
 Coelho-de-Souza, Lívia Noronha, et al. "Relaxant effects of the essential oil of Eucalyptus tereticornis and its main constituent 1, 8-cineole on guinea-pig tracheal smooth muscle." Planta medica 71.12 (2005): 1173-1175. [Link]