Is strep the only thing that causes the tonsils to be red?

No: Anything that causes irritation or inflammation of the tonsils can cause them them to be red. Viruses (such as typical cold viruses) are actually a more common cause of "red" tonsils than strep bacteria. Allergies, with mucus dripping down the back of the throat, also causes redness of the throat and tonsils.

No: Tonsil inflammation and redness can occur due to bacterial and/or viral infections. When a person has "strep throat" this refers to a bacterial infection due to a particulat type of streptococcus bacteria. Red tonsils can occur with viral infections too, such as with mono. A throat culture can help determine if the tonsillitis is due to strep.