I've copied this below from a facebook post. Unfortunately it wasn't attributed to anyone which is a shame how these things go around. Anyway I thought it was interesting. I personally would try the first part of it and ignore the advice in the second part (about not saving onion leftovers).
Wow- very interesting....
ONIONS! I had never heard this!!!
PLEASE READ TO THE END: IMPORTANT
In 1919 when the flu killed 40 million people there was this Doctor that visited the many farmers to see if he could help them combat the flu...
Many of the farmers and their families had contracted it and many died.
The doctor came upon this one farmer and to his surprise, everyone was very healthy. When the doctor asked what the farmer was doing that was different the wife replied that she had placed an unpeeled onion in a dish in the rooms of the home, (probably only two rooms back then). The doctor couldn't believe it and asked if he could have one of the onions and place it under the microscope. She gave him one and when he did this, he did find the flu virus in the onion. It obviously absorbed the bacteria, therefore, keeping the family healthy.
Now, I heard this story from my hairdresser. She said that several years ago, many of her employees were coming down with the flu, and so were many of her customers. The next year she placed several bowls with onions around in her shop. To her surprise, none of her staff got sick. It must work. Try it and see what happens. We did it last year and we never got the flu.
Now there is a P. S. to this for I sent it to a friend in Oregon who regularly contributes material to me on health issues. She replied with this most interesting experience about onions:
Thanks for the reminder. I don't know about the farmer's story...but, I do know that I contacted pneumonia, and, needless to say, I was very ill... I came across an article that said to cut both ends off an onion put it into an empty jar, and place the jar next to the sick patient at night. It said the onion would be black in the morning from the germs...sure enough it happened just like that...the onion was a mess and I began to feel better.
Another thing I read in the article was that onions and garlic placed around the room saved many from the black plague years ago. They have powerful antibacterial, antiseptic properties.
This is the other note. Lots of times when we have stomach problems we don't know what to blame. Maybe it's the onions that are to blame. Onions absorb bacteria is the reason they are so good at preventing us from getting colds and flu and is the very reason we shouldn't eat an onion that has been sitting for a time after it has been cut open.
LEFT OVER ONIONS ARE POISONOUS
I had the wonderful privilege of touring Mullins Food Products, Makers of mayonnaise. Questions about food poisoning came up, and I wanted to share what I learned from a chemist.
Ed, who was our tour guide, is a food chemistry whiz. During the tour, someone asked if we really needed to worry about mayonnaise. People are always worried that mayonnaise will spoil. Ed's answer will surprise you. Ed said that all commercially-made mayo is completely safe.
"It doesn't even have to be refrigerated. No harm in refrigerating it, but it's not really necessary." He explained that the pH in mayonnaise is set at a point that bacteria could not survive in that environment. He then talked about the summer picnic, with the bowl of potato salad sitting on the table, and how everyone blames the mayonnaise when someone gets sick.
Ed says that, when food poisoning is reported, the first thing the officials look for is when the 'victim' last ate ONIONS and where those onions came from (in the potato salad?). Ed says it's not the mayonnaise (as long as it's not homemade mayo) that spoils in the outdoors. It's probably the ONIONS, and if not the onions, it's the POTATOES.
He explained onions are a huge magnet for bacteria, especially uncooked onions. You should never plan to keep a portion of a sliced onion.. He says it's not even safe if you put it in a zip-lock bag and put it in your refrigerator.
It's already contaminated enough just by being cut open and out for a bit, that it can be a danger to you (and doubly watch out for those onions you put in your hotdogs at the baseball park!). Ed says if you take the leftover onion and cook it like crazy you'll probably be okay, but if you slice that leftover onion and put on your sandwich, you're asking for trouble. Both the onions and the moist potato in a potato salad, will attract and grow bacteria faster than any commercial mayonnaise will even begin to break down.
Also, dogs should never eat onions. Their stomachs cannot metabolize onions.
Please remember it is dangerous to cut an onion and try to use it to cook the next day, it becomes highly poisonous for even a single night and creates toxic bacteria which may cause adverse stomach infections because of excess bile secretions and even food poisoning.
Please pass this on to all you love and care about.