Ayin horah is mentioned numerous times in talmudic texts.
There is a difference of opinion as to how much we should be concerned about it (see below)
It can affect anyone with devastating results.
It can have a negative effect on the physical wellbeing of a person or even their financial or social or mental state.
According to the Ba'al Shem Tov- facial/skin blemishes are caused by Ayin horah!
According to Rav (Bava Metziah 107b) ninety nine out of a hundred people who die, have died from ayin horah. According to Rashi, (bava Metziah 107b) Rav is of the opinion that all illnesses are caused by ayin horah!
The Shulchan Aruch gives the reason for not calling up two brothers consecutively to the Torah- because of ayin horah. (O"H' 141' 66')
The removal of ayin horah can allow medical treatment to be successful, or to be avoided altogether. Removal can dissolve the barriers that prevent success in any area of life.
We can (usually) only know for sure how ayin horah is affecting someone after identifying it during the procedure of removal and then witnessing substantial positive changes in their life.
Often however, during the procedure of removing ayin horah (pouring the lead- see 'Treatment' at top of main page) we can identify what part of the body the ayin horah is affecting. For example, whilst treating a woman (she was not even present during the procedure) who requested the procedure because she was waiting years for a shidduch, we identified from the 'lead procedure' that she was suffering from lower-back pain that was affecting her ability to walk,- that she subsequently confirmed to be true. (see 'Success stories' on main menu bar, and "The Lead Reveals!" for more recent examples).
Sheva Tiheyenoh, does not ever attempt to predict the future from the lead - only to reveal how ayin horah may be currently affecting a person!
Sheva Tiheyenoh has -with 'siyyatah dishmayah'- successfully identified and/or treated problems with shoulders, legs, arms, teeth, female hair loss, mental issues, shidduchim, migraines, parnossah, -even someone who claimed they were 'possessed' by a foreign 'spirit', among other things, as well as people who claimed they were simply pursued by 'bad luck'.
In the experience of Sheva Tiheyenoh, there has been almost no one who was treated by the procedure- who didn't show clear signs that they had ayin horah...
How concerned should we be about Ayin Horah?
There seems to be a machlokes (argument) amongst gedolei Yisroel whether we should be concerned about ayin horah:
Rav Moshe Feinstein ztz''l (Even Ha'ezer 3' 26) writes that one need not be overly concerned with ayin horah as it only affects people who are worried about it.
Rav Eliezer Papoe ztz''l (the Peleh Yoetz who is quoted regarding other inyonim by the Chofetz Chaim ztz''l in Mishnah Brurah) however writes (in siman 'ayin horah') that 'everyone should be indeed concerned about ayin horah and to take precautions against ayin horah is begeder ''Venishmartem Lenafshosechem"!!'