The US Centers for Disease Control and Prevention (CDC) issued an updated version of health service guidelines for health workers incorporating advances in therapy. Risk of infection through transmission of the HIV is rare for workers who are exposed to it. Documented cases of those who acquired the infection involved sharp objects such as needle sticks. Precautionary measures include written protocols and access to clinicians. Treatment is only done with one drug, zidovudine. Combination of drug regimens is recommended against resistant virus strains.