Why do some people who are considered healthy, have massive heart attacks with no warning or symptoms? is this very common?

Heart Attacks: Heart attacks are uncommon in young men and women age less than 40 to 45 years. Younger patients with this tend to have a family history of it. Cigarettes are the most common associated risk factor. Other risk factors for young people are cocaine use, Factor V Leiden deficiency and oral contraceptives. Obesity and diabetes are less common but still contribute to risk for heart attacks.