That investigation led to the imprisonment of Bonds' personal trainer as well the head of BALCO, the lab south of San Francisco where Bonds had occasionally undergone blood and urine testing.

The BALCO scandal disgraced top athletes in track and field, baseball and football after evidence showed that doping fueled their athletic achievements.

Track-and-field champion Marion Jones recently admitted taking banned drugs and was disqualified from all competition since Sept 1. 2000, including the Sydney Olympics where she won five medals. Tim Montgomery, once dubbed the world's fastest man, saw his 100-meter record stripped and was barred from competition in 2005.

Bonds, never the most popular of athletes because of an abrasive personality, has long denied doping. Yet many fans have suspected that steroids powered Bonds when he set the single-season home-run record in 2001 and kept him going strong at an age when many of his peers lose strength and endurance.

Bonds passed Hank Aaron's Major League Baseball career home record -- perhaps the greatest mark in American sport -- this season and finished seven homers ahead at 762. Yet his long-time team, the San Francisco Giants, chose not to offer him a contract for the 2008 season and his future in the game remains in doubt.

DIFFICULT TO PROVE

Perjury, which is knowingly lying in a judicial proceeding while under oath, is often difficult to prove and is not often prosecuted, legal experts say.

In one precedent from the world of sports, Chris Webber, a basketball player with the Detroit Pistons, was charged with perjury and in 2003 pleaded guilty to criminal contempt of court. He avoided time in jail in the scandal over payments to players while he was at the University of Michigan.

The government has gathered information from Bonds' friends, lovers and associates, although his personal trainer spent many months in prison for refusing to testify.

Kimberly Bell, one of Bonds' lovers who testified, recently told Reuters he once admitted using steroids. Another potential prosecution witness is a former close friend and business partner who feuded with Bonds.

The indictment also says officials obtained evidence of positive tests for steroids and other performance-enhancing substances.

Supporters of the record seven-time baseball Most Valuable Player say federal agents have proved overzealous in their prosecution of the case.

"The amount of government resources that have been devoted to this -- you know, chasing people that Barry may have had an extramarital affair for a week or something -- it's really a misplacement of emphasis and resources," Bonds business attorney Laura Enos told Reuters last year.

Copyright © 2007 Reuters Limited. All rights reserved. Republication or redistribution of Reuters content, including by framing or similar means, is expressly prohibited without the prior written consent of Reuters. Reuters shall not be liable for any errors or delays in the content, or for any actions taken in reliance thereon. Reuters and the Reuters sphere logo are registered trademarks and trademarks of the Reuters group of companies around the world.

Study shows how Chikungunya virus spread so far

WASHINGTON - A new virus called the Chikungunya virus, which causes painful and sometimes crippling symptoms, has spread to several new countries in the past year because it has found a new species of mosquito to carry it, researchers said on Friday.

A single mutation allowed the virus to infect the Asian tiger mosquito -- which itself is spreading to many more countries in Europe and North America, the researchers said.

"This mutation increases the potential for Chikungunya virus to permanently extend its range into Europe and the Americas," Stephen Higgs and colleagues at the University of Texas Medical Branch wrote in their report, published in the Public Library of Science journal PLoS Pathogens.

This is especially true if average temperatures continue to rise with global warming, they wrote. The virus caused outbreaks in India and Italy this year.

Chikungunya is a type of virus called an arbovirus and was carried mostly by the Aedes aegypti mosquito. It caused an epidemic that began in Kenya in 2004 and spread to several Indian Ocean islands including the Comoros, Mauritius, the Seychelles, Madagascar, Mayotte and Reunion.

On tiny Reunion Island alone more than a third of the population -- 266,000 people -- was infected, with debilitating aches and pains. It killed 260 people.

But because Aedes aegypti mosquitoes are not found in Reunion, researchers suspected that something else was carrying the virus.

Knowing that the virus that caused the Reunion outbreak had mutated, the researchers tested it to see if that mutation gave the virus the ability to infect other mosquito species.

They tried to infect various species, including the Asian tiger mosquito, Aedes albopictus, with genetically engineered strains of the virus and found that viruses with the very simple mutation thrived in the tiger mosquitoes.

"This research gives a new insight into how a simple genetic change in a human pathogen can increase its host range and therefore its geographic distribution," they wrote.

"Aedes albopictus is abundant and widely distributed in urban areas of Europe and the United States of America, and this work suggests that these areas are now vulnerable to Chikungunya establishment."

Copyright © 2007 Reuters Limited. All rights reserved. Republication or redistribution of Reuters content, including by framing or similar means, is expressly prohibited without the prior written consent of Reuters. Reuters shall not be liable for any errors or delays in the content, or for any actions taken in reliance thereon. Reuters and the Reuters sphere logo are registered trademarks and trademarks of the Reuters group of companies around the world.