The human genome is littered with the genetic remains of ancient viruses that once infected people but now lie dormant. Until recently, scientists didn't believe they played a role in modern disease.

New research is causing many scientists to think again. Recent studies suggest these old virus shards may play a role in Hodgkin's lymphoma, multiple sclerosis, rheumatoid arthritis and other diseases.

The ancient viruses scientists are most interested in date from waves of infections that took place as recently as 100,000 to 200,000 years ago. In modern humans, the genes were most likely inherited from a common ancestor infected with retroviruses, probably from rodents. Unlike other kinds of viruses, retroviruses copy their own genes into a host's DNA. Some of the retroviruses are believed to have infected sperm cells and eggs, ensuring the virus would be passed from generation to generation in the genome.

Researchers estimate that 8 percent of the human genome consists of virus genes and their remains, translating into approximately 80,000 genes scattered in an individual's DNA. That's about twice as many as the number of genes that determine a person's height, eye color, hair color and other characteristics, said John M. Coffin, a retrovirologist at Tufts University, who is studying these ancient virus genes. "There is more virus in us than there is us in us,'' he said.

Animal species including mice have ancient retrovirus genes implicated in diseases. It wasn't until 2003, when the human genome was published, that researchers realized humans have old virus genes, too. The emergence of huge databases containing gene sequences from animals and humans allowed researchers to start mining for possible connections between old viruses and present-day diseases. 

Click here to read more from The Wall Street Journal.