"When I was fifteen, I got hepatitis"
 Hepatitis is an illness characterised by inflammation of the liver.  It is usually caused by viral infection, but can also develop as a result of bacteria, alcoholic or fungal poisoning, or as a severe side effect to certain medications.  In its acute form, as with Michael's illness, recovery may take up to six months and is primarily achieved through rest.  After this time, hepatitis is classed as chronic.  In very severe cases it can lead to cirrhosis or cancer of the liver. 

For in depth information about the illness see the WHO fact sheet.