Why some Children get repeated infections?

In my daily pediatric practice, I see many children with repeated infections especially common cold , that presents with cough, dry or phlegmy, runny nose, may be sore throat, sometimes diarrhea, with fever or no fever at all . Many of these children are attending daycare or school where there is a close physical contact among children which favors the transmission of infectious diseases especially upper respiratory tract infection and increased risk of chest infection and gastroenteritis or maybe hepatitis A or even some parasitic infestation. Most of these infections are viral, at least in the beginning and the period of sickness last for average of seven to ten days, they mostly get well with no antibiotics .The question which is usually asked by parents is whether or not the child has some sorts of immune deficiency.

Apart from being in contact with other sick children, age is another factor, and the younger they are especially less than 3 years of age, there is more chance of getting repeated infections. There are of course other contributing factors, like poor nutrition, environment, anatomical defect or allergy but the chance of immune deficiency is much less. However, the warning signs of immune deficiency are as follows:
1) Six or more ear infections within one year
2) Two or more sinus infections in one year
3) Two or more months on antibiotics with little effects
4) Two or more pneumonia infections within one year
5) Failure of the infant to gain weight and grow normally
6) Recurrent deep skin or organ abscesses
7) Persistent fungal infections in the mouth or skin after one year
8) Two or more deep seated infections such as meningitis, osteomyelitis, cellulitis or sepsis
9) Family history of primary immune deficiency