Young children under 3 years have an immature (weak) immune system which is unable to control severe infections. The younger the child, the less mature the immune system.

Some older children have an immune system which has been weakened by illness, malnutrition or drugs:

• Children with HIV infection
• Children recovering from measles or whooping cough
• Malnourished children
• Children on large doses of steroids

Severe tuberculosis itself weakens the immune system.