Spyware vs. viruses: Two different fights

Malware, whether it be spyware, adware or viruses, has been the focus of many recent studies conducted by the media, universities and vendors. And despite their differences, the studies consistently show that anti-virus programs can achieve 100% efficacy ratings, while spyware prevention programs can only achieve about 90% efficacy. So what makes spyware so different from viruses? Why is perfect prevention possible for viruses but not spyware? The answer is far from simple, and it involves an understanding of how uninvited software can take up residence on a PC. It also involves differentiating viruses, with the urges to reproduce from spyware, which harvests user information and secretly shares it with third parties. In general, however, what makes these two large classes of malicious software different also helps to explain why one may be somewhat easier to corral and detect than the other.