Zombie computers.

That you being a doctor do not know what a DDoS is, just reflects the lack of literacy on ICT and the failure of your Faculty to bring you the adequate knowledge for the safety practice of Medicine in the XXI century.

In any case here is an explanation. Two type of attachs are possible

  1. DoS = Denial of Services. This is an attach of Denial of Services (see below) localized.
  2. DDoS= Distributed Denial of Services. Is an attach distributed all over.

On this situation a malaware virus transform your computer in a BOTMASTER or Zombi computer that sends an enormous quantity of messages to the server that reacts blocking all the traffic. This means that block not only the bad traffic but the good messages (who might be essential in a situation of distant healthcare). The problem is that your computer collects an army of bots searching for other non-protected computers to introduce the same malaware. At the end you get a BOTNET army capable of infecting and attacking hundreds of servers and blocking the Internet traffic.