To warn.
The sentence aims to give notice of the danger, impending evil, possible harm, or unfavorable consequence of having a strep throat. Although it does provide helpful information to the reader, the information is very limited to achieve the purpose of educating the reader about the strep throat (it does not say anything specific about what it is, the causes, the symptoms, the specific consequences nor the treatment).
Instead, it focuses on telling the reader that it may be a serious affliction if one is not cautious. Therefore, it warns us about the complications of the strep throat, so we could be more cautious.