Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

INTRODUCTION TO PATHOGENS AND THE HUMAN MICROBIOTA
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Plasmodium falciparum causes the most serious form of malaria and is the most
intensively studied of the malaria-causing parasites. It exists in many distinct
forms, and it requires both the human and mosquito hosts to complete its sexual
cycle (Figure 23–9). Several of these forms are highly specialized to invade and
replicate in specific tissues—the lining of the insect gut, the human liver, and the
human red blood cell. Even within a single host cell type, the red blood cell, the
Plasmodium parasite undergoes a complex sequence of developmental events,
reflected in striking morphological changes (Figure 23–9B–D).
All Aspects of Viral Propagation Depend on Host Cell Machinery
Bacteria, fungal, and protozoan pathogens are living cells themselves. They use
their own machinery for DNA replication, transcription, and translation, and, for
the most part, they provide their own sources of metabolic energy. Viruses, by
contrast, are the ultimate hitchhikers, carrying little more than information in the
form of nucleic acid. Most clinically important human viruses have small genomes
consisting of double-stranded DNA or single-stranded RNA (Table 23–1), and we
now have complete genome sequences of almost all of them.
Viral genomes typically encode three types of protein: proteins for replicating
the genome, proteins for packaging the genome and delivering it to more
host cells, and proteins for modifying the structure or function of the host cell to
enhance the replication of the virus (see Figure 7–62). In general, viral replication
involves (1) entry into the host cell, (2) disassembly of the infectious virus particle,
(3) replication of the viral genome, (4) transcription of viral genes and synthesis of
viral proteins, (5) assembly of these viral components into progeny virus particles,
Table 23–1 Viruses That Cause Human Disease
Virus Genome type Disease
Herpes simplex virus 1 Double-stranded DNA Recurrent cold sores
Epstein–Barr virus (EBV) Double-stranded DNA Infectious mononucleosis
Varicella-zoster virus Double-stranded DNA Chickenpox and shingles
Smallpox virus (Variola) Double-stranded DNA Smallpox
Human papillomavirus Double-stranded DNA Warts, cancer
Adenovirus Double-stranded DNA Respiratory disease
Hepatitis-B virus Part single-, part double-stranded DNA Hepatitis B
Human immunodeficiency virus (HIV-1) Single-stranded RNA [+] strand Acquired immune deficiency syndrome (AIDS)
Poliovirus Single-stranded RNA [+] strand Poliomyelitis
Rhinovirus Single-stranded RNA [+] strand Common cold
Hepatitis-A virus Single-stranded RNA [+] strand Hepatitis A
Hepatitis-C virus Single-stranded RNA [+] strand Hepatitis C
Yellow fever virus Single-stranded RNA [+] strand Yellow fever
Coronavirus Single-stranded RNA [+] strand Common cold, respiratory disease
Rabies virus Single-stranded RNA [–] strand Rabies
Mumps virus Single-stranded RNA [–] strand Mumps
Measles virus Single-stranded RNA [–] strand Measles
Influenza virus type A Single-stranded RNA [–] strand Respiratory disease (flu)