Type 1 diabetes occurs when the body is unable to produce insulin. Insulin is a hormone that is needed to control the amount of glucose (sugar) in your blood.
When you eat, your digestive system breaks down food and passes its nutrients into your bloodstream.
The pancreas (a small gland behind your stomach) usually produces insulin, which transfers any glucose out of your blood and into your cells, where it is converted to energy.
However, if you have type 1 diabetes, your pancreas is unable to produce any insulin (see below). This means that glucose cannot be moved out of your bloodstream and into your cells.
Type 1 diabetes is an autoimmune condition. Your immune system (the body's natural defence against infection and illness) mistakes the cells in your pancreas as harmful and attacks them, destroying them completely or damaging them enough to stop them producing insulin.
It is not known exactly what triggers the immune system to do this, but some researchers have suggested that it may be due to a viral infection.
Type 1 diabetes is usually inherited (runs in families), so the autoimmune reaction may also be genetic.
If you have a close relative, such as a parent, brother or sister with type 1 diabetes, you have about a 6% chance of also developing the condition. The risk for people who do not have a close relative with type 1 diabetes is just under 0.5%.