כו  וַיֹּאמֶר אִם-שָׁמוֹעַ תִּשְׁמַע לְקוֹל יְהוָה אֱלֹהֶיךָ, וְהַיָּשָׁר בְּעֵינָיו תַּעֲשֶׂה, וְהַאֲזַנְתָּ לְמִצְו‍ֹתָיו 
 וְשָׁמַרְתָּ כָּל-חֻקָּיו–כָּל-הַמַּחֲלָה אֲשֶׁר-שַׂמְתִּי בְמִצְרַיִם, לֹא-אָשִׂים עָלֶיךָ, כִּי אֲנִי יְהוָה, רֹפְאֶךָ.      26
 and He said: ‘If thou wilt diligently hearken to the voice of the LORD thy God, and wilt do that which is right in His eyes, and wilt give ear to His commandments, and keep all His statutes, I will put none of the diseases upon thee, which I have put upon the Egyptians; for I am the LORD that healeth thee.’ {S} 

From  this pasuk we may learn that refua [medicine] should be preventive medicine!!
otherwise: 
…..I will put none of the diseases upon thee, which I have put upon the Egyptians; for I am the LORD that healeth thee.’ {S}
 would make no sense.
If G-d would not put diseases upon us, then we would need not:
for I am the LORD that healeth thee.’
Conclusion: because I am the LORD rofecha [and thus will prevent diseases from you]……….,therefore non of these diseases will come upon us……………….This is "preventive medicine" in the real sense.
I heard this from my teacher z"l who brought this in the name of the Rambam.