Some viral and bacterial pathogens can cause or contribute to chronic, life-threatening illnesses that are not normally classified as infectious diseases. An important example is cancer. As discussed in Chapter 20, the oncogene concept—that certain altered genes can trigger cell transformation and tumor development— came initially from studies of the Rous sarcoma virus, which causes a form of cancer (sarcomas) in chickens. One of the viral genes encodes an overactive homolog of the host tyrosine kinase Src (see Figure 3–63), which has been implicated in many kinds of cancer. Several human cancers are also known to have a viral origin. Human papillomavirus, for example, which causes genital warts, is responsible for more than 90% of cervical cancers (see Figure 20–40). The recent development of a vaccine against the most abundant cancer-associated strains of human papillomavirus promises to prevent many of these cancers in the future. In other cases, chronic tissue damage caused by infection can increase the likelihood of cancer. Inflammation caused by the stomach-dwelling bacterium H. pylori can be a major contributor to stomach cancer, as well as to gastric ulcers.
The major causes of death in wealthy industrialized nations are cardiovascular diseases. They frequently result from atherosclerosis, the accumulation in blood vessel walls of fatty deposits that can block blood flow and cause heart attacks and strokes. A hallmark of early atherosclerosis is the appearance in blood vessel walls of clumps of macrophages called foam cells, which recruit other white blood cells into the forming atherosclerotic plaque. Foam cells in atherosclerotic plaques often contain the bacterial pathogen Chlamydia pneumoniae, which commonly causes pneumonia in humans and is a significant risk factor for atherosclerosis in humans and animal models. Other bacterial species are also implicated in atherosclerosis, including bacteria usually associated with teeth and gums, such as Porphyromonas gingivalis. As we learn more about the interactions between pathogens and the human body, it seems likely that more chronic conditions will be found to have a link to an infectious agent.
Pathogens Can Be Viruses, Bacteria, or Eukaryotes
Many types of pathogens cause disease in humans. The most familiar are viruses and bacteria. Viruses cause diseases ranging from AIDS and smallpox to the common cold. Viruses are essentially fragments of nucleic acid (DNA or RNA) that generally encode a relatively small number of gene products, wrapped in a protective shell of proteins (Figure 23–2A) and (in some cases) an outer membrane envelope (see Figure 5–62). Much larger and more complex than viruses, bacteria are prokaryotic cells, which perform most of their basic metabolic functions themselves, relying on the host primarily for nutrition (Figure 23–2B).
Some other infectious agents are eukaryotic organisms. These range from single-celled fungi and protozoa (Figure 23–2C) to large, complex metazoa such as parasitic worms. One of the most common human parasites, shared by about a billion people at present, is the nematode worm Ascaris lumbricoides, which infects the gut (Figure 23–2D). It closely resembles its harmless nematode cousin Caenorhabditis elegans, which is used as a model organism for genetic and developmental biological research (see Figure 1–39). C. elegans, however, is only about 1 mm in length, whereas Ascaris can reach 30 cm.
We now introduce the basic features of each of the major types of pathogens, before we examine the mechanisms that pathogens use to infect their hosts.
Figure 23–2 Pathogens in many forms.
The structure of the protein coat, or capsid, of poliovirus. This virus was once a common cause of paralysis, but the disease (poliomyelitis) has been greatly reduced by widespread vaccination.