In people bitten by Venom A Mojave rattlesnakes (those outside the relatively small Venom B area in south-central Arizona), the onset of serious signs and symptoms can be delayed, sometimes leading to an initial underestimation of the severity of the bite. Significant envenomations (as with all snakebites, the quantity of venom injected is highly variable and unpredictable) can produce vision abnormalities and difficulty swallowing and speaking. In severe cases, skeletal muscle weakness can lead to difficulty breathing and even respiratory failure.[17] Contrary to popular belief, fatalities are uncommon. (Wikipedia)

Pero de que duele duele y que es fatal puede serlo ciertamente.