@lisa270988 We are all coming in to contact with circulating viruses every day. As adults, we have been encountering them since we were born and have developed immunity to most of them. That is why most elderly adults are rarely troubled with colds - over the years they've seen them all so it takes something new to affect them.
Children are born with some immunity that they were given in the womb. As they become older, that maternal immunity wears off and they start building their own, through the infections they acquire through being with other children. As they grow, they become immune to things they have already encountered.
When they perhaps leave home and live around other young people in university halls or similar, there is another surge of infection - fresher's flu - when new viruses from areas other than the home one, are encountered. Again, the young people quickly become immune to them, as they did as young children.
Now clearly, @lisa270988, you have a fear of normal childhood illnesses and it is not entirely irrational because you probably fear for your family's health. Try to reassure yourself that this is a normal part of child development and essential for your DC's future health. Take normal precautions at home with hand washing and wiping down surfaces if there are DV bugs around, but be prepared for illness to happen and try to stay relaxed about it. It will be far more harmful to DC in the long run, if you try to prevent what is normal priming of their immune system (and whatever measures you take will probably not work anyway). You can do this ❤️