It hardly seems credible, but in fact there are several studies showing that aspirin (yes, good ol’ acetylsalicylic acid!) can help prevent plant viruses.
If a potential insect vector (most plant viruses are transported from plant to plant by leaf-piercing insects such as leafhoppers, aphids, thrips, whiteflies, etc.) appears on your plants, it might be wise to spray them lightly with water containing aspirin until the insect is under control. The recommended dose is one uncoated aspirin tablet in one gallon (4 liters) of water, also adding a few drops of insecticidal soap to the mix to help the solution better adhere to the foliage.
Of course, if you think it over, the idea shouldn’t be all that surprising. After all, plants naturally produce salicylic acid, the precursor for aspirin, when attacked by insects or diseases. It’s a defence mechanism that tells the plant’s immune system it is time to activate. The difference is that, in theory, by applying diluted acetylsalicylic acid before the attack, it is you who is now warning the plant to get ready for the coming invasion.
Note that, until now, this preventive effect has mostly been officially tested under laboratory conditions… but many amateur gardeners trying it have claimed it works very well, so it’s still an idea worth considering.