1.Underlying medical conditions
2.Chronic heart disease, such as heart failure.
3.Chronic kidney disease.
4.Chronic liver disease, such as hepatitis.
5.Chronic neurological conditions, such as Parkinson's disease, motor neurone disease,...
6.Diabetes.
7.Problems with the spleen, such as sickle cell disease or...
8.A weakened immune system as the result of conditions such...
9.Being seriously overweight, with a body mass index of 40...