Past a certain point, memorizing the definition of words is kind of a waste of time.  Dictionaries do the best they can, but frequently leave out many subtle nuances, as well as commonly associated words or phrases. 

Take your example of "virulence".  The dictionary won't tell you that the noun form is much lesson common than the adjective "virulent", which is frequently paired with *disease*.  This is due to the original meaning of the word, from the Latin:

> [**virulent**][1] (adj) : c. 1400, in reference to wounds, ulcers, etc., "full of corrupt or poisonous matter," from Latin *virulentus* "poisonous," from *virus* "poison" (see virus). Figurative sense of "violent, spiteful" is attested from c. 1600. 

From this etymology we can see that virulent literally means "like a virus".  Example:

> In the mid 14th century a **virulent** plague known as the "Black Death' swept through the Middle East and Europe, killing tens of millions.

It would be difficult to use *virulent* without implying a similar kind of pervasive and detrimental condition.  You might say for example, that I have a "virulent rage" toward some person, to imply it is akin to a vicious disease throughout my whole body.

This is what Tᴚoɯɐuo means in his comment by "figurative meaning".  The second definition is figurative, and based on the literal first definition.  

Of course, this doesn't mean you can apply any word in any situation.  As with any language, the key to understanding how to use all the "big words" is to *read extensively*, and see how they appear in context.  Also, like any language, English has a certain rhythm and flow, so that certain words simply sound better when paired with other words, or when placed in a particular order.  

Writers also frequently play with things like *contrast* and *tone*.  It's pretty clear what I might mean by *virulent anger*, but what do you think I would imply if I wrote:

> He burned with a **virulent passion** for her, so much so that ...

In any case: If you read a lot, eventually the words will become individuals with their own unique personalities.  You might also browse the etymology dictionary (in the above link) to find more about the origin of certain words, including prefixes, stems, and suffixes, and how these (sometimes) relate to other similar-sounding English words.  In this way you might be able to guess at the meanings and nuances of new words you read.


  [1]: https://www.etymonline.com/word/virulent#etymonline_v_7824