You know they tested less than 1.4% of the total people onboard (45 tested of 3534 on board), right? And 21 of those tests, or 46% came back positive. From the very limited information given, the tests were done on crew members with symptoms and the 15 or so passengers who were on both the Mexico Cruise and the Hawaii Cruise. The odds that "only 2 passengers" were infected are incredibly small. The infections are most likely in the hundreds.
Your point about the vast majority of cruises actually having ZERO infections is true, at this early stage. That will change. My sister is 10 days into an 18 day cruise right now. She loves cruising and wasn't going to miss this one over the small number of ships having problems.
That is exactly why this virus is so risky...it spreads fast. The crew on Grand Princess should have been better screened. Last time I read the news about that ship, 17 infected were crew and 2 were passengers. The crew of the ship I was on 12 days ago, any crew member who had traveled to China recently was filtered out. And all passengers were questioned about recent travels and their passports looked at. Like I said in other posts, there were 2400 or so cruises within last 2 months. If only 2 or 3 had problems, that is pretty good ratio.
That will of course change as more are tested from Grand Princess. What is the silver lining with this virus is that most healthy people are not dying from it.