H.G. Wells was wrong! It wasn't the germs that killed the poor martians but too much salt:
Early Mars 'too salty' for life
"The Red Planet was too salty to sustain life for much of its history, according to the latest evidence gathered by one of the US rovers on Mars' surface. High concentration of minerals in water on early Mars would have made it inhospitable to even the toughest microbes, a leading NASA expert says.
Clues preserved in rocks that were once awash with water suggest the environment was both acidic and briny. The observations were made by the US space agency's Opportunity rover. It has spent months examining rocks on an ancient Martian plain."