A processor of a medical device configured to communicate with a remote server can be programmed to protect the medical device from exposure to unauthorized or malicious software. A system or method to implement this form of protection can include, for example, at least one processor on the medical device, a control software module that controls the operation of the medical device and is executable on the processor, a data management module that manages data flow to and from the control software module from sources external to the medical device, and an agent module that has access to a limited number of designated memory locations in the medical device. In addition, a hemodialysis apparatus can be configured to operate in conjunction with an apparatus for providing purified water from a source such as a municipal water supply or a well. A system for controlling delivery of purified water to the hemodialysis apparatus can comprise a therapy controller of the hemodialysis apparatus configured to communicate with a controller of a water purification device, and a user interface controller of the hemodialysis apparatus configured to communicate with the therapy controller, and to send data to and receive data from a user interface.