Yeah, you didn't read the article....
So to spell it out. Bacteria lives on seaweed (and is able to eat it directly, and get energy from it). Japanese eats the seaweed, and thus the bacteria. The bacteria in the intestines swap notes with their cousins. Now part of the population in the intestine can do the digesting the seaweed trick.
We have bacteria in and on our bodies. This bacteria outnumbers the cells we call "us". Most of this bacteria actually does a job, such as reducing inflammation around broken skin, or helping you digest your food.
In the article, they were talking about gene transfer between bacteria living on the seaweed, and bacteria now found to be living in the intestines of some Japanese. Furthermore, current research suggests that humans "inherit" their population of intestinal flora from their mothers. So nowhere did anyone talk about phages, or any other virus. Accept you.
I am saddened that a sufficient volume of accurate information, completely off topic, is "insightful".