Diabetes mellitus refers to a group of diseases that affect how your body uses blood sugar (glucose). Glucose is vital to your health because it's an important source of energy for the cells that make up your muscles and tissues. It's also your brain's main source of fuel.
The underlying cause of diabetes varies by type. But, no matter what type of diabetes you have, it can lead to excess sugar in your blood. Too much sugar in your blood can lead to serious health problems.
Chronic diabetes conditions include type 1 diabetes and type 2 diabetes. Potentially reversible diabetes conditions include prediabetes when your blood sugar levels are higher than normal, but not high enough to be classified as diabetes and gestational diabetes, which occurs during pregnancy but may resolve after the baby is delivered.

Types of Diabetes
Type 1 Diabetes
Type 1 diabetes occurs because the insulin-producing cells of the pancreas (called beta cells) are destroyed by the immune system. People with type 1 diabetes produce no insulin and must use insulin injections to control their blood glucose. Type 1 diabetes most commonly starts in people under the age of 20, but may occur at any age

Type 2 Diabetes
Unlike people with type 1 diabetes, people with type 2 diabetes produce insulin. However, the insulin their pancreas secretes is either not enough or the body is unable to recognize the insulin and use it properly. When there isn't enough insulin or the insulin is not used as it should be, glucose can't get into the body's cells. Type 2 diabetes usually occurs in people over age 40 who are overweight, but can occur in people who are not overweight.
Sometimes referred to as "adult-onset diabetes," type 2 diabetes has started to appear more often in children because of the rise in obesity in young people.
Gestational Diabetes
Gestational diabetes is triggered by pregnancy. Hormone changes during pregnancy can affect insulin's ability to work properly. The condition occurs in approximately 4% of all pregnancies.Pregnant women who have an increased risk of developing gestational diabetes are those who are over 25 years old, are above their normal body weight before pregnancy, have a family history of diabetes

Symptoms
Diabetes symptoms vary depending on how much your blood sugar is elevated. Some people, especially those with prediabetes or type 2 diabetes, may not experience symptoms initially. In type 1 diabetes, symptoms tend to come on quickly and be more severe.
Some of the signs and symptoms of type 1 and type 2 diabetes are:
Increased thirst
Frequent urination
Extreme hunger
Unexplained weight loss
Presence of ketones in the urine (ketones are a byproduct of the breakdown of muscle and fat that happens when there's not enough available insulin)
Fatigue
Irritability
Blurred vision
Slow-healing sores
Frequent infections, such as gums or skin infections and vaginal infections