The topic position refers to the information provided at the beginning of a sentence. This information serves two functions for a reader. First, it should introduce to the reader what information will be presented in the sentence.
We mentioned previously that each sentence should discuss one idea—the topic position should introduce this idea. To make this new idea familiar to the reader, it needs to link back to previously discussed information. That is the second function of the topic position—to serve as a topic link.
Avian influenza infection rates have been increasing worldwide. Transmission has been rapid owing to high levels of international travel. H5N1 is one type of avian influenza currently being studied. Epidemiology studies have shown this virus to be especially pathogenic.