The immune system is responsible for protecting us from infection, so a weakened immune system will lead to a heightening in viral and bacterial infections. These can be mild or sever depending on your overall immune health.
A weak immune system can lead to more frequent and persistent coughs and colds, but can also allow infections to escalate into conditions such as pneumonia, meningitis and bronchitis. People with low immune function may also be more prone to autoimmune disorders, digestive issues, and developmental issues (particularly in children and young people).