You do not need anti-virus software to protect a linux operating system. But you could do windows users a favour by installing anti-virus on a linux box to scan files you got from windows users which you intend to send to other windows users i.e. protecting windows users from each other.
The way linux is built, making viruses and malware for it is very hard and the effort will not pay off to just attack the average joe (as in the case of windows).
Attacks against linux systems will focus on select high-profile targets where investing the huge time and effort to hack in to them pays off with money, data, goods.
No hacker will want to invest hours, days, sweat and effort, to hack in to your linux box to steal your family fotos, regardless of how many people start using linux.
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