We may also be aware of some minor types like LADA (latent autoimmune diabetes of adults), a slower-onset form of autoimmune diabetes that appears in adults. Another type is MODY (maturity-onset diabetes of the young), which often, but not always, appears in children or young people. It is monogenic, meaning a mutation in only one gene causes the disease, and the affected gene is dominant, meaning that you can inherit only one copy of the gene from one parent and a normal gene from the other parent and you'll get MODY.
Type 2 diabetes is polygenic, meaning a lot of genes are usually involved.
Other forms of diabetes include gestational diabetes, a usually temporary form of diabetes that occurs in late pregnancy and then goes away after the baby is born, although having gestational diabetes greatly increases your risk of getting type 2 diabetes later. And there are various rare forms of diabetes as well as diabetes induced by taking drugs, including steroids.
But there's another form of diabetes that seems to becoming more and more common, especially in Africa, and that's Flatbush diabetes, named after the New York area where it was described some years ago.
It's also been called a lot of other things, including atypical diabetes, type 1B diabetes, idiopathic [meaning the cause is unknown) type 1 diabetes, and ketosis-prone type 2 diabetes.
I'll use the term Flatbush diabetes because I find it easier to rememember colorful nicknames, and also because it doesn't require one to decide if this form of diabetes is really type 1 or type 2. In fact, it seems to be somewhat in between.
Flatbush diabetes is found primarily in nonwhite populations, especially those of sub-Saharan African descent, although Asians, Hispanics, and even Caucasians can be diagnosed with it.
The hallmark of Flatbush diabetes is presenting with sudden onset of extremely high BG levels (BG levels of 700 mg/dL and A1c's of 12 to 14 are not uncommon), in diabetic ketoacidosis, or DKA. Because DKA used to be considered a hallmark of type 1 diabetes (in older terminology, type 2 diabetes was sometimes called ketosis-resistant diabetes), patients with Flatbush diabetes were originally considered to be type 1, and hence the name atypical type 1 or type 1B (regular type 1 diabetes is classified as type 1A according to the current official classification system).
However, patients with Flatbush diabetes don't have antibodies against beta cells; unlike "regular" type 1 diabetes, it's not an autoimmune disease. This is what it was sometimes called idiopathic type 1 diabetes.
Patients with Flatbush diabetes are also often overweight, as in type 2 diabetes, and have relatives with type 2 diabetes, and they may have some insulin resistance, which is one reason some people use the term type 1.5 diabetes. I think that term type 1.5 should be avoided, because it's sometimes used for LADA and sometimes used for type 1 patients who also show signs of insulin resistance.