Security Aspects

I'm now developing a web based application (Dental Practice Management System) for the customers. This application should be able to cater for the multiple locations accessing,
which means the database server, the webserver will be located at the main clinic only. Then the clinic branches (more than one) can access in.
I'm using jsp,mysql and tomcat. I wish to find the security aspects of data transmission in a multiple locations environment. How easy is it for unauthorized users to steall the data(e.g Patient Data) when there is data transmissoon? How to enhance the security? Please advise.
Thank you.