SBDs are just steem wrapped in a smart contract that converts each SBD to one US dollar worth of steem when ever the conversion function is invoked.

Steem Dollars are Steem presented in a different package. So they are not infinite. Furthermore, if at any point the issuing of steem dollars gets to more than 10% of the total supply the blockchain will automatically stop the "printing" of SBDs.

Although, technically the Steem's supply is not hard capped. The inflation rate is set to decline over time but never stop. So you can say that the creation of steem is "infinite" but the creation of steem dollars depends on the market price of steem. The higher the price of steem the higher the print rate of steem dollars with a cap of 10% of the total supply in USD terms.