Why do you think that pirated software is far more likely to be infected with viruses than original software distributed by manufacturers?

Answer 1

If by "who" you actually mean"why" then my answer would be the following:
Because pirated software is known to be used by hackers to spread their viruses through the download of said software. Where as original software distributed by manufacturers is unlikely to contain viruses as (one would hope) the manufacturers would avoid infecting their customers computers as they probably want you to buy more software from them in future.
However, original, licensed software is all too often susceptible to infection by viruses (even Microsoft's software). It is also true that large corporations will do anything within the law to increase revenues and this may include bundling "Malware" such as Adware with their original licensed software or introducing said Malware at ISP level to avoid detection by Anti Virus software (PLEASE SEE http://www.telegraph.co.uk/technology/news/4974695/Online-privacy-demanded-by-web-founders.html). In summary I would say that Pirated Software is more likely to contain Malware but this is becoming less true as big business begins to cash in on what "Hackers" have been doing since the dawn of the age of computers. The proliferation of Malware by big business is inevitable as they follow our governments in forcing us to (in the unlikely event that they give us a choice) "opt-out" of being spied upon in all aspects of our every day lives in the pursuit of profits.

Answer 2

because it is not copyrighted and therefore no security is available.