Each form of diabetes has a different primary cause. However, diabetes can result in an excess of sugar in the blood regardless of the type you have. Serious health issues can result from an excess of sugar in the blood.

Each form of diabetes has its own unique causes.

1. Type 1 diabetes - The precise cause of type 1 diabetes is still unknown. The immune system wrongly targets and kills insulin-producing beta cells in the pancreas for some unknown cause. Some people may be affected by their genes and an immune system attack may be triggered by a virus.
2. Type 2 diabetes - It stems from a combination of genetics and lifestyle factors. Being overweight or obesity increases your risk, too. The effects of insulin on your blood sugar are resisted by your cells more when you are overweight, especially in the abdomen. This condition runs in families. Family members have genes that increase their risk of type 2 diabetes and obesity. 
3. Gestational diabetes - It is brought on by hormonal changes that take place during pregnancy. The placenta secretes hormones that reduce the sensitivity of a pregnant person's cells to the effects of insulin. Pregnancy-related elevated blood sugar can result from this. Gestational diabetes is more likely to develop in people who are overweight before becoming pregnant or who put on too much weight while pregnant.

Treatment of diabetes
Several different drugs are used by doctors to treat diabetes. Some are ingested, while others can be obtained via injections.

• Type 1 diabetes: Insulin is the primary medication used to treat type 1 diabetes. It takes the place of the hormone that your body is unable to make. People with type 1 diabetes frequently utilize a variety of insulin kinds. Their effects are different in terms of how long they persist and how quickly they begin to operate.
• Type 2 diabetes: Some people can manage type 2 diabetes with diet and exercise. You'll need to take medicine if changing your lifestyle isn't enough to lower your blood sugar levels. One or more of these medications may not be enough for you. Insulin is used by some type 2 diabetics.
• Gestational diabetes: If you are diagnosed with gestational diabetes, you must check your blood sugar levels frequently each day while you are pregnant. If it's high, dietary adjustments and exercise may be sufficient to lower it.

According to research, between 15% and 30% of pregnant women who acquire gestational diabetes will require insulin to control their blood sugar. The developing baby is safe when using insulin.

Diabetes and diet
An essential component of treating diabetes is a healthy diet. In some circumstances, altering your diet may be sufficient to control the condition.

• Type 1 diabetes: Depending on the meals you eat, your blood sugar level rises or lowers. Foods that are starchy or sugary cause a sharp spike in blood sugar levels. Increases from protein and fat happen more gradually. Your medical team might advise you to restrict the number of carbohydrates you consume each day. Additionally, you must balance your insulin dosages with your carbohydrate intake.
• Type 2 diabetes: Eating the appropriate meals will help you control your blood sugar levels as well as shed any extra pounds. A crucial component of eating for type 2 diabetes is carb counting. You can calculate how many grams of carbohydrates to consume at each meal with the aid of a dietician.
• Gestational diabetes: It's critical for both you and your unborn child to eat a healthy diet during these nine months. You can prevent the need for diabetes medications by making the appropriate meal choices.

Limit foods high in sugar or salt, and watch your portion sizes. You should refrain from consuming too much sugar, even if you need some to feed your developing child.

Diabetes and exercise
Exercise is crucial for managing diabetes, along with food and therapy. All forms of diabetes are consistent with this.
Maintaining an active lifestyle improves the way your cells respond to insulin and lowers blood sugar levels.


Reference:
https://www.healthline.com/health/diabetes#causes