[kisielk]

I'm sure Apple has the resources to hire enough sysadmins to keep their systems up to date. The question is why it isn't happening.

[czhiddy]

It looks like the exploit used a vulnerability in Apache Struts that was revealed on 7/16 : https://news.ycombinator.com/item?id=6081428

Given that, your question of "how can we completely avoid zero-day attacks" is nonsensical.