Explosive growth in sales, or spread of product information through customer contact (referrals). The term was coined by the US venture capital firm Draper, Fisher, Jurveston.
http://www.businessdictionary.com/definition/viral-marketing...

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Note added at 1 hr (2010-07-20 07:04:04 GMT)
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this is a metaphoric expression; a figure of speech in which a word or phrase that ordinarily designates one thing is used to designate another, thus making an implicit comparison.
والمقصود هو انتشار المنتج وتغلغله بين أوساط المستهلكين أو العملاء على نطاق واسع والتشبيه بالأمراض الفيروسية واسعة الإنتشار

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