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Since there is dead and weak sperm the possible cause is due to presence of interfereing substance after sperm in formed in the testis. You need to take medicine which has Coenzyme Q10
which is known to prevent asthenospermia and improve the movement and strength of sperm. Also avoid any heat coming near testis like standing near hot oven, taking hot bath, keeping laptop on lap etc. Eat plenty of fresh fruits and green leafy vegetables. Drink plenty of water. Exercise daily by brisk walking for 40 minutes as all these reduce damaging effects of free radicals on body and sperm as well. Keep in follow up with your Doctor. Repeat sperm test after 90 days of start of effective treatment.