Most children develop the full potential of their immune system by getting in contact with the viruses and bacteria in the period between the age of two and four. However, if the infections come too often (once every month, as you reported) then you might want to visit an immunologist to see if there is any immunological disorder.
The doctor might be able to give or rule out a possible diagnose based on the physical exam, anamnesis you give and the laboratory blood tests.