A few DUCKDUCKGO hits revealed that others believe there may be something to garlic and onion in relation to colds and flu.
"Onions contain the same compound, allicin, that gives garlic its famous infection-fighting qualities, according to pharmacist J. Albert Hermle, author of the book "Finally!!! Stop Having Colds & Flu."
Allicin has the ability to slow and kill a variety of viruses and bacteria."
DO GARLIC and ONIONS KILL FLU AND COLD VIRUSES?
Other internet hits I looked at also say it may help. So based on your grandfather's experience, I will add some things to my diet along with some other things that sound like good ideas.
Who knows, there is a possibility that he was right about his "theory!"
Legal Disclaimer: There is nothing in this post that contains proof that Garlic might increase survival rates of people who are exposed to the coronavirus. Therefore, use at your own risk. All democrats (especially Bernie Sanders supporters) are urged to NOT eat garlic or onions so that they can be part of our experiment. Results of survival rates of conservatives eating garlic vs. fatality rates of leftists who did not eat garlic will be published by 2030 in Scientific American /sarc