Growing up way back when, playing in mud, catching frogs and the like built me a strong immune system.
I have gone through my life without any allergies and I am certain it is because we were encouraged to be outdoors exploring, getting dirty. Playing in the mud. Allergies have been on the increase over the last 30 years, especially in urban areas. Allow your children to get out there and get dirty.
Outside world bacteria is definitely good for health. If you stay indoors so long it affects you. Bacteria like anything else can be useful or harmful.
Thats not really how it works. An allergy is an overreaction of your immune system to something relatively unharmful. It's genetic and can be triggered by exposure to certain types of chemicals. For example, the chemicals they use to peel of floors or something of that nature is prone to cause allergic reactions, and can give you permanent allergies.
@MartinTheFirst I will stick with my theory. Growing up in the country, having a large family, exposed me to a fair amount of microbes, thus enabling my immune system to build tolerance and to differentiate good from bad. Reacting more, building tolerance to the harmful bacteria/microbes. 🙂