It’s remarkable how the terpene has become such a figurehead in everyday modern culture these days, considering that terpenes are in many types of well-known plants beyond cannabis. Essential oils are comprised of terpenes extracted from flowers or herbs or spices, to name a few. It’s definitely no secret that terpenes are highly medicinal as the scientific literature continuously bolsters and validates this notion. 
Despite their beneficial properties for humans and insects like honeybees, terpenes can prove deadly against nefarious destructive creatures like spider mites or aphids, bacteria, and different types of viruses.
For example, a 2009 study screened star anise oil, which is rich in trans-anethole, as well as isolated trans-anethole, eugenol, beta-caryophyllene, caryophyllene oxide, beta-eudesmol, and farnesol for antiviral activity against herpes simplex virus (HSV).  The oil and isolated terpenes/terpenoids all reduced infectivity from levels of 40% to >99%. The trans-anethole rich oil and isolated beta-caryophyllene revealed the highest selectivity indices, which measures the ratio between toxicity to host cells and anti-viral properties (therefore, the higher the value, the more effective and safer the substance would be). These researchers had previously demonstrated similar results using several other botanical extracts, including peppermint, tea tree, and thyme essential oils. [3,4]
A 2020 review article provides a wealth of citations of other studies using essential oils for treatments against HSV-1 besides those mentioned above (including hyssop, sandalwood, lemon balm, wormwood, cypress, lavender, lemongrass, juniper, and many more). 
Functional mushrooms are also showcasing their strong medicinal promise. Triterpenes extracted and isolated from the mushroom species called Ganoderma pfeifferi demonstrated potent efficacy against herpes simplex virus. 
A 2012 study gauged the efficacy of different Brazilian plants for activity against rotavirus, a gastrointestinal virus common in infants and younger children.  Among the 14 plants evaluated, the top performers showing the strongest activity against rotavirus included leaf extracts from Byrsonima verbascifolia (a shrub that bears edible fruits), Eugenia dysenterica (a tree with edible fruits that can act as a laxative), Hymenaea courbaril (West Indian locust), and Myracrodruon urundeuva (a timber tree used in beekeeping). Unfortunately, these researchers didn’t quantify the specific ingredients native to each extract that resulted in antiviral properties but noted that other studies have pointed to terpenoids (and flavonoids) as inhibiting viral invasion. 
Other plants have been evaluated for inhibiting infection from hepatitis C virus (HCV), for which a vaccination does not exist.  Bupleurum roots (from a plant resembling fennel) have been used in traditional Chinese medicine for over two millennia, and crude extracts, which include triterpenoid saponins, have demonstrated antiviral properties against influenza in addition to HCV.  Specifically, saikosaponin b2 has been shown to engage with HCV and block its entry into host cells. 
So, it should come as no surprise, then, that several studies have focused on the effectiveness of essential oils and terpenes therein against different coronaviruses. [5,10] Essential oils extracted from Nigella sativa (black cumin, kalonji), Anthemis hyalina (like chamomile) and Citrus sinensis (sweet oranges like navel or blood oranges) were shown to limit the replication of coronaviruses.  This past body of literature has sparked interest in terpene studies aimed at combating our current coronavirus pandemic.
Hallowed be thy terpene.
- Lupoi J. The Cannabis Terpene Experience. Pismo Beach: Mace Media Group; 2020.
- Astani A, Reichling J, Schnitzler P. Screening for antiviral activities of isolated compounds from essential oils. Evid Based Complement Alternat Med. 2011;2011:253643. [journal impact factor = 1.813; times cited = 141 (SemanticScholar)]
- Schuhmacher A, Reichling J, Schnitzler P. Virucidal effect of peppermint oil on the enveloped viruses herpes simplex virus type 1 and type 2 in vitro. Phytomedicine. 2003;10(6-7):504-510. [journal impact factor = 4.268; times cited = 191 (SemanticScholar)]
- Schnitzler P, Koch C, Reichling J. Susceptibility of drug-resistant clinical herpes simplex virus type 1 strains to essential oils of ginger, thyme, hyssop, and sandalwood. Antimicrob Agents Chemother. 2007;51(5):1859-1862. [journal impact factor = 4.715; times cited = 109 (SemanticScholar)]
- Nadjib, B. Effective antiviral activity of essential oils and their characteristic terpenes against coronaviruses: an update. Journal of Pharmacology & Clinical Toxicology. 2020;8(1):1138. [journal impact factor = N/A; times cited = N/A]
- Niedermeyer TH, Lindequist U, Mentel R, et al. Antiviral terpenoid constituents of Ganoderma pfeifferi. J Nat Prod. 2005;68(12):1728-1731. [journal impact factor = 3.779; times cited = 87 (SemanticScholar)]
- Cecílio AB, de Faria DB, Oliveira Pde C, et al. Screening of Brazilian medicinal plants for antiviral activity against rotavirus. J Ethnopharmacol. 2012;141(3):975-981. [journal impact factor = 3.690; times cited = 67 (SemanticScholar)]
- Lin LT, Chung CY, Hsu WC, et al. Saikosaponin b2 is a naturally occurring terpenoid that efficiently inhibits hepatitis C virus entry [published correction appears in J Hepatol. 2015 Jul;63(1):292]. J Hepatol. 2015;62(3):541-548. [journal impact factor = 20.582; times cited = 50 (SemanticScholar)]
- Yang F, Dong X, Yin X, Wang W, You L, Ni J. Radix Bupleuri: A review of traditional uses, botany, phytochemistry, pharmacology, and toxicology. Biomed Res Int. 2017;2017:7597596. [journal impact factor = 2.276; times cited = 37 (SemanticScholar)]
- Ulasli M, Gurses SA, Bayraktar R, et al. The effects of Nigella sativa (Ns), Anthemis hyalina (Ah) and Citrus sinensis (Cs) extracts on the replication of coronavirus and the expression of TRP genes family. Mol Biol Rep. 2014;41(3):1703-1711. [journal impact factor = 1.703; times cited = 15 (SemanticScholar)]