The basic Google search assumes AND between each word.
For example, medical science breakthroughs will find sites with all three words. Use OR if you want either of two terms e.g., medical science breakthroughs OR discoveries.
To better focus your search:
Search specific sections of sites for better precision:
Sadly, Google has retired the tilde (~) for synonym searching and the plus sign (+) to insure a word is included in your search.