If it occurs after 7 days of age, it is most likely caused by a virus. If it occurs after 21 days of age, it is most likely due to coccidiosis. After saying this, it is always important to work closely with your veterinarian to make a diagnosis. If you don't know the cause, you don't know what to do to prevent future calves from getting it, and also what drugs and treatments are required. There are diseases that can be prevented by the use of vaccines.
Vaccination is like insurance.
You would not gamble by not buying fire insurance for your house even though you haven't had a fire for the past 5 years. The same is true with vaccinating for scours, because sooner or later, even with good management, the problem will arise. The cost is very small compared to the alternative. The loss of one calf in 100 covers the cost of the vaccine. The cost of treating a calf even if it lives, is often high, not to say anything about the work involved.