prepositions - The difference between 'protect from' and 'protect against'

The Longman dictionary suggests two options regarding the word 'protect': protect somebody/something from something and protect somebody/something against something/

Examples:
The cover protects the machine from dust.
Physical exercise can protect you against heart disease.

According to the examples, I believe that the verb 'protect' takes
against' when you are talking about something which is not tangible, whereas the preposition 'from' is used with words that can be seen or touch. Am I correct? If not, how to know which preposition is necessary?