11. A method of modifying content in a document, comprising: comparing original content in a document received from an original content provider with a stored user profile to identify at least a first portion of original content that is undesirable based upon the user profile, wherein the user profile includes medical information about a user; requesting a second portion of personalized content comprising health-related information from a substitute i 0 document server based upon said medical information about said user; generating a modified version of the document by modifying at least the first portion of the original content in the document with the second portion of personalized content received from the substitute document server in response to the request; and providing the modified version of the document to a user set that displays the modified version of the document to the user, wherein the first portion of the original content in the document that is modified is selected based upon an identifier in the original content.
11. A method of modifying content in a document, comprising: comparing original content in a document received from an original content provider with a stored user profile to identify at least a first portion of original content that is undesirable based upon the user profile, wherein the user profile includes medical information about a user; requesting a second portion of personalized content comprising health-related information from a substitute i 0 document server based upon said medical information about said user; generating a modified version of the document by modifying at least the first portion of the original content in the document with the second portion of personalized content received from the substitute document server in response to the request; and providing the modified version of the document to a user set that displays the modified version of the document to the user, wherein the first portion of the original content in the document that is modified is selected based upon an identifier in the original content. 17. The method of claim 11 , wherein the first portion of the original content in the document that is modified is selected based upon an address in the original content.