Before the Iraq war, pathogenic bacteria known to the medical community as Acinetobacter baumannii attracted little attention. After all, Acinetobacters are a common soil microbe found in the United States and in other parts of the world. A. baumannii is ordinarily a commensal bacterium, one that may live on the skin of healthy people for many years without ever causing disease. So, most people exposed to A. baumannii do not become sick or colonized by them. But hundreds of cases have been reported in recent years, among them large numbers of wounded soldiers in field hospitals in Iraq. The patients who died were shown to have a strain of bacteria, resistant to most drugs, that can survive even on dry, nonliving surfaces for up to two months, making it difficult to control and prevent infections from spreading.

A. baumannii is what is known as an opportunistic pathogen. It only infects hosts with compromised immune systems. These include patients taking immunosuppressive therapies, those with immune disorders, and people who have endured enormous stress, as from battle wounds. If it spreads unchecked, A. baumannii can cause death through high fevers, pneumonia, meningitis, or infections of the spine or blood (sepsis). As many as three out of four infected people have died from it, so it's highly virulent. Even for survivors, the effects of infection can be extreme, including the loss of limbs.

While most bacteria will die in the presence of an antibiotic, especially if the patient has never taken that particular antibiotic before, some bacteria have mutations that allow them to survive even in the presence of an antibiotic. When antibiotic-resistant bacteria reproduce, the identical copies they make of themselves are also resistant. As bacteria can reproduce exponentially—going from one to a billion overnight—the proportion of resistant bacteria is greatly increased. Once established, resistance is hard to eliminate. It may even spread to different species of bacteria.

A genomic analysis revealed that A. baumannii has about 50 resistance genes. In a laboratory setting, it has developed resistance to new drugs in as few as 12 hours. Spontaneous mutations in genomic DNA allow organisms to acquire new characteristics. However, these DNA changes usually occur in increments and typically involve the loss, rather than gain, of a function. Microbes can also change more rapidly under environmental stress by acquiring DNA through a process known as horizontal gene transfer, or HGT. HGT refers to mating processes through which an organism incorporates genetic material from another organism, allowing for gene exchange. The antibiotic-resistant strains of A. baumannii resulted from nonresistant strains that over time acquired genes via HGT for greater resistance and more virulence.

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