Chronic (long-term) respiratory diseases, such as asthma, chronic obstructive pulmonary disease, emphysema or bronchitis.

Chronic heart disease, such as heart failure.

Chronic kidney disease.

Chronic liver disease, such as hepatitis.

Chronic neurological conditions, such as Parkinson's disease, motor neurone disease, multiple sclerosis (MS), a learning disability or cerebral palsy.

Diabetes.

Problems with the spleen, such as sickle cell disease or if removal of the spleen has occurred.

A weakened immune system as the result of conditions such as HIV and AIDS, or medicines such as steroid tablets or chemotherapy.

Being seriously overweight, with a body mass index of 40 or above.