Popular encrypted messaging app and Facebook (FB  ) subsidiary WhatsApp has transitioned from being a casual form of social media communication for the private use of friends and family to a workplace communication option. Other rapid messaging apps, like Slack and Microsoft (MSFT  ) Teams, are also increasingly favored over phone, email, or in-person meetings.

These workplaces include hospitals. For busy doctors, messaging apps are a fast and effective way to communicate and collaborate over patient care. But WhatsApp and many other messaging apps aren't specifically designed for the secure sharing of highly sensitive medical information.

That's where Dutch startup Siilo comes in. Over the past 2.5 years, Siilo has become the go-to app designed specifically for use in medical settings. The app comes with features like in-app discussion groups, patient note storage, and sharing capabilities designed to ensure exchange of confidential data in full compliance with HIPAA confidentiality laws. Siilo is growing rapidly, and recently earned €4.5 million in funding. Since its launch in May of 2016, Siilo has amassed more than 100,000 users, with more than 7.5 million messages processed and 6,000 active clinical chat groups each month.

"People all around the world use WhatsApp to connect with small businesses they care about - from online clothing companies in India to auto parts stores in Brazil," Siilo stated. "But WhatsApp was built for people and we want to improve the business experience. For example, by making it easier for businesses to respond to customers, separating customer and personal messages and creating an official presence."

Messages sent via Siilo are also automatically deleted after 30 days unless the user specifies a particular message should be retained. The app has done away with automated back-ups of users' conversations, so copies can't be made. Other useful features include the ability to blur images, augment images with arrows for emphasis, and export threaded conversations to electronic health records. The app can only be opened with a PIN code or fingerprint and facial recognition, and comes with an auto remote wipe functionality in the event of theft.

Still, Siilo is just one player in an increasingly crowded field. "Our main competitor is always WhatsApp," says Siilo CFO Arvind Rao. "Obviously there are also other players trying to move in this space. TigerText is the largest in the US. In the UK we come across local players like Hospify and Forward...A major difference we have very experienced in-house dev team...The experience of this team has helped to build a messenger that really can compete in usability with WhatsApp that is reflected in our rapid adoption and usage numbers."

Rao added, "We haven't come across any other secure messenger for healthcare in Europe with these figures in the App Store/Google Play rankings and therefore believe we are the largest in Europe...We have multiple large institutions across Western Europe where doctors are using Siilo."