Nowadays E-health cloud systems are more and more widely employed. However the security of these systems needs more consideration due to the sensitive health information of patients. So far, some protocols about secure e-health cloud systems have been proposed, but many of them use the traditional PKI infrastructure to implement cryptographic mechanisms, which is cumbersome as they require every user having and remembering its own public/private keys. Identity based encryption (IBE) is a cryptographic primitive, which uses the identity information of the user (e.g., email address) as the public key. Hence, the public key is implicitly authenticated and the certificate management is greatly simplified. On the other hand, proxy re-encryption is a cryptographic primitive aiming at transforming a ciphertext under the delegator A's into another ciphertext, which can be decrypted by the delegatee B. In this paper, we describe several identity related cryptographic techniques for securing an E-health system, which include new IBE schemes and new identity based proxy re-encryption (IBPRE) schemes. We also prove these schemes' security and give their performance analysis. Our results show that our IBPRE scheme is especially highly efficient for re-encryption, which can be used to achieve cost-effective cloud usage.