"Water, the main culprit"
The flu is not only an air born virus, but has the ability to travel by means of liquids and other organisms. Different species are usually the instigator of viruses, just as the world's water source is a great mechanism of travel.
Not a day goes by that we do not use water, so imagine if our water supply was contaminated by a strain of the virus. Water contamination is more often times than not our fault. We do not know how to properly discard household waste, so that most of the time it ends up in our water supply system.
Like any elementary science class will tell you, water is recyclable. So when the waste from your household enters the water system, it is then used to feed the plants, then you eat the plants. So even though you think you are ridding your household of harmful viruses, it is actually only being recycled into your food and being ingested again. Its a vicious cycle, but if you cut it off from the get go, then you can ensure your safety and those around you.