KINDS OF DISEASE
Diseases are categorized and addressed based on general etiology: infectious, genetic, and multifactorial. These categories are overlapping and, at times, blurry.1
Infectious diseases are caused by pathogenic microbes that (1) colonize, (2) sense/adapt to, (3) persist in, and (4) escape from the hosts they infect. Genetic determinants that facilitate pathogenesis are called virulence factors. Of note, microbial surface proteins attach to (and in some cases enter) host cells, transcription factors turn on/off virulence gene expression in response to environmental cues, and secreted proteinaceous toxins sabotage the normal function of host cells. Virulence factors that belong to these three broad categories are often the targets of intense study and vaccine-based prevention. Common biotechnologies used to study virulence factors include comparative genetics, DNA microarray, quantitative reverse transcriptase-polymerase chain reaction (qRT-PCR), reporter fusions, western blots, enzyme-linked immunosorbent assays (ELISAs), immunofluorescent microscopy,2 and gene knockouts made by rDNA technologies.
Host characteristics also play a critical role in the manifestation of disease. These include the genetic susceptibility and immunological strength of the individual. HIV is particularly problematic because it destroys the host immune system as it replicates, making infected individuals susceptible to any number of secondary infections. Pathogens that exclusively cause disease in immunocompromised hosts are called opportunistic pathogens3 and may be carried asymptomatically at high rates in the general population. Asymptomatic carriers complicate infectious disease eradication efforts.
Biotechnologies to treat infectious diseases are designed to manipulate pathogenic relationships; by blocking or turning off microbial virulence factors and enhancing or replacing the host immune response. Historically, antibiotics have played a significant role in the management of bacterial infections. Unfortunately, these drugs are becoming less effective as pathogen resistance emerges due to the natural genetic plasticity of microbes and overuse of antibiotics in household products and agriculture. Some new antimicrobials aim to promote nonpathogenic lifestyles of bacteria, rather than killing them with broad strokes. The behavior-modification strategy exerts less selective pressure on microbial populations to become resistant. Unfortunately, insufficient attention is placed on the development of new antimicrobials,4 despite the dire need. See Box 3.1 for more on this.