How to counter Karthus
Let us take a look on how to beat Karthus. We will look at champions who are strong against Karthus and who are weak against Karthus. Use win rate, gold difference and creep score difference at minute 10 to find the best 
Jungle champion who counters Karthus. You can already win in champion select with the right counters for Karthus. If you want to know how to counter Karthus then you need to take into account the best picks vs Karthus and the worst picks vs Karthus. This Karthus counter data is based on matches played by Pros on patch 13.24 and previous ones. It is important to see whether Pros counter picked Karthus or whether they played as Karthus.
Champions used by Pros to counter pick Karthus
Looking at the matches from Pros (see table Pros counter picked Karthus) we can see that they picked 
Lee Sin most often to counter pick Karthus. The matchup Lee Sin vs Karthus occured 32 times. Karthus has 14 wins and 18 losses in this matchup. Learn how to counter pick Lee Sin to get another perspective on the data.
Lane outcome for Karthus
When looking at Karthus counters you will notice that win rates alone are not conclusional. Therefore we provide an overview (see table Lane Outcome) where you can see the lane outcome of Pros who played Karthus. The lane outcome of Karthus consists of three outcomes: Win, Loss or Draw. Let's take a look of Karthus lane and match outcome for specific matchups. Considering the lane outcome and the match outcome of a specific matchup, might enable you to find the right counter for Karthus.
Conclusion
Analyze the available data and learn how to beat Karthus. To master Karthus take also a look on Karthus builds and runes for Karthus. Good luck, Summoner!