Okay guys we're not talking apt-get here.
It's about advanced persistent threat.
Didn't find anything about it after using the search.
What is it in short?
An advanced persistent threat (APT) is a network attack in which an unauthorized person gains access to a network and stays there undetected for a long period of time. The intention of an APT attack is to steal data rather than to cause damage to the network or organization. APT attacks target organizations in sectors with high-value information, such as national defense, manufacturing and the financial industry .
Why do a lot of APT campaigns get to the public so late?
It's connected to the average life expectancy.
Zero-days have an average life expectancy of nearly seven years, with a quarter surviving over nine years. The median amount of time it takes to create an exploit for a known vulnerability is 22 days 
If you want to read up on the aftermath of different APT's, their motivation, detection and how cleaning up afterwards works check the links below.