AD is not contagious. AD inflammation results from too many reactive inflammatory cells in the skin. Research is seeking the reason why these cells over-react. When something triggers them, they don't turn off as they should. Trigger factors may be different for different people.
Most children get worse when they get a cold or other infection.
The skin's main function is to provide a barrier against dirt, germs and chemicals from the outside. We don't notice this barrier unless it gets dry, and then it's scaly, rough and tight. People with AD have a defect in their skin and it won't stay moist. It is especially bad in winter when the heat is on in the house and the humidity drops. Other things that dry the skin are too much bathing without proper moisturizing.