Dutch mobile game publisher Spil Games has a hit with Operate Now: Hospital, a surgery game that has been downloaded more than 10 million times in six months.

The surgery simulator is available on iOS and Android, and most of the downloads have been organic, or through word of mouth rather than paid advertising. The hospital-building and surgery game has been successful thanks to its dramatic storylines, according to Amsterdam-based Spil Games.

Above: Operate Now; Hospital has dramatic stories.

Image Credit: Spil Games

The downloads have been driven by the surgery, which focus on realistic simulation and resource management. After the first day, 50 percent of players return to the game. That’s a pretty high retention rate for a mobile title.

“10 million is a modest success, but it is definitely a nice milestone for us,” said Tung Nguyen-Khac, Spil Games’ CEO, in a statement. “It demonstrates a building momentum in our strategic move into mobile gaming. This is our first mobile game with both serious install numbers and significant revenue potential.”

Spil Games said 95 percent of the 10 million downloads have come from word of mouth. Players have spent an average of 25.8 minutes a day playing the game. The top three countries on iOS are the United Kingdom, France, and the U.S. 78 percent of iOS users play on the iPhone, and 21 percent on the iPad.

Above: Operate Now; Hospital lets you build your own hospital.

Image Credit: Spil Games

In time for the holidays, the game has gotten a major update. Spil Games has added new surgery simulations that provide fresh challenges for players. Players can add a new floor of rooms to their hospitals, and the stories have been extended. Players can also add holiday decorations.

Overall, Spil Games has had more than 275 million installs for all of its games in the past two years, across the web and mobile platforms. Spil Games also has a long-term partnership with Nolan Bushnell, the godfather of video games, to develop his new mobile intellectual properties.