Increased dangers of eye irritation and infection are caused by sharing contact lenses with others.
First of all, the borrower of contact lenses have increased risk of eye abrasion, because the contact lenses he borrowed from the owner has been fitted well with the owner and won't be fitted well with the borrower.
Secondly, the borrower also has a higher risk of allergies to not only contact lenses, but also the solutions and other related items. After all, one man's meat is another's poison.
Lastly, the bacteria from the origin wearer will possibly be transferred to the second wearer, resulting in eye infection.