Plants have evolved various defence mechanisms including antiseptic, antibiotic and antifungal components. Some of these are collected in so-called "essential oils" and these oils can certainly have an impact. Perhaps the best known is tea tree oil. Some essential oils are toxic in high concentrations and some others seem merely to smell nice.
As for Thieves essential oil, which seems to be a mixture of Clove, Lemon, Cinnamon Bark, Eucalyptus and Rosemary, the commercial website points a single published study of this particular mixture, in 1998 in the Journal of Essential Oil Research which does show some impact but does not seem to imply it is a cure-all. There is rather more in other publications on the individual components.
As I said in a comment, the Thieves website has a disclaimer
These statements have not been evaluated by the Food and Drug
Administration. These products are not intended to diagnose, treat,
cure, or prevent any disease
which I think suggests that you should not rely too heavily on it preventing all forms of disease.