Type the words "public health" (without quotes), select the words, click
on Tools | AutoCorrect Options, type the abbreviation you would like
Word to automatically replace with the selected words, and click on OK.

xio wrote:

if i am typing the words "public health" repetitively throughout the
document, is there a way i can avoid repetitive typing? for example, typing
p.h. will automatically transform to the word public health.