One of the most serious and intractible medical problems facing doctors today is that of antibiotic-resistant infectious microorganisms, the so-called "superbugs." One example is MRSA (methicillin-resistant Staphylococcus aureus), now a common source of blood poisoning and infection in hospitals (see sidebar below). Other exampless of antibiotic-resistant infectious agents include Bacillus anthracis Sterne, virulent Escherichia coli, Klebsiela pneumoniae, Helicobacter pylori, and Mycobacterium terrae, viruses with lipid membranes, and a number of invasive fungi.