At the time of this writing, Dropbox uses standard SSL encryption when transferring data between your computer and the Dropbox servers, and according to Dropbox, the data is encrypted when stored on their servers. MIT does not have the resources to independently validate or refute this claim, and we encourage you to refer to Dropbox's documentation on the matter: Dropbox - How secure is Dropbox?
One thing to note is that when using the Dropbox client, your Dropbox data appears to your operating system as just another folder, indistinguishable from, e.g. My Documents or Desktop This means it is susceptible to anything that modifies data on your local computer. For example, malware such as CryptoLocker could affect your Dropbox folder just as it affects any other data stored on your computer. Similarly, if someone gains access to your computer, they will likely be able to access your Dropbox folder.
When using the Dropbox client, it is particularly important to ensure that your anti-virus software is up to date, that your operating system is configured to take automatic updates, and that you are using Strong Passwords on your computer.