additional equipment, software, and manpower needed to combat the problem. Spam's direct effects include the consumption of computer and network resources, and the cost in human time and attention of dismissing unwanted messages. Large companies who are frequent spam targets utilize numerous techniques to detect and prevent spam. The cost to providers of search engines is significant: "The secondary consequence of spamming is that search engine indexes are inundated with useless pages, increasing the cost of each processed query". The costs of spam also include the collateral costs of the struggle between spammers and the administrators and users of the media threatened by spamming. Email spam exemplifies a tragedy of the commons: spammers use resources (both physical and human), without bearing the entire cost of those resources. In fact, spammers commonly do not bear the cost at all. This raises the costs for everyone. In some ways spam is even a potential threat to the entire email system, as operated in the past. Since email is so cheap to send, a tiny number of spammers can saturate the Internet with junk mail. Although only a tiny percentage of their targets are motivated to purchase their products (or fall victim to their scams), the low cost may provide a sufficient conversion rate to keep the spamming alive. Furthermore, even though spam appears not to be economically viable as a way for a reputable company to do business, it suffices for professional spammers to convince a tiny proportion of gullible advertisers that it is viable for those spammers to stay in business. Finally, new spammers go into business every day, and the low costs allow a single spammer to do a lot of harm before finally realizing that the business is not profitable. Some companies and groups "rank" spammers; spammers who make the news are sometimes referred to by these rankings. General costs In all cases listed above, including both commercial and non-commercial, "spam happens" because of a positive cost-benefit analysis result; if the cost to recipients is excluded as an externality the spammer can avoid paying. Cost is the combination of Overhead: The costs and overhead of electronic spamming include bandwidth, developing or acquiring an email/wiki/blog spam tool, taking over or acquiring a host/zombie, etc. Transaction cost: The incremental cost of contacting each additional recipient once a method of spamming is constructed, multiplied by the number of recipients (see CAPTCHA as