The belief that donating blood is bad for your health is just a silly misconception. On the contrary in the case of a healthy person, donating blood once a year is a good thing. Donating blood is only a problem if you're badly malnourished, underweight, have a low blood count, suffer from blood pressure problems etc. in which case you will not be allowed to donate blood eitherways.
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There are many situations in which patients need blood to stay alive for example,
a patient needs blood after a major accident in which there is loss of blood.
No major surgery is performed without blood as there is bound to be blood loss.
On an average, for every open heart surgery about 6 units of blood is required.
In miscarriage or childbirth, cases the patient may need large amount of blood to be transfused for saving her life and also the child's. For patients with blood diseases like severe Anaemias especially Aplastic Anaemias, Leucaemias (blood cancer), Haemophilia (bleeding disorder), Thalassemia etc. repeated blood transfusions are the only solution. In many other situations like poisoning, drug reactions, shock, burns, blood transfusion is the only way to save precious human life. So if we can save someone's precious life by donating few drops of blood, it can make a big difference!