Figure 1. Four viruses: (a) polio, (b) influenza, (c) herpes, and (d) HI (human immunodeficiency) virus. The relative sizes are actually quite different: (a) 28 nm in diameter, (b) 120 nm, (c) 200 nm, and (d) 100 nm. Composition: (a) single-stranded RNA in an icosahedron; (b) eight single-stranded RNA segments surrounded by proteins and a membrane with hemagglutinin and neuraminidase molecules; (c) double-stranded DNA in an icosahedron surrounded by a membrane with glycoproteins; and (d) two single-stranded RNA molecules, reverse transcriptase, and two other enzymes in a capsid surrounded by a membrane with proteins. (Watercolor and gouache: Anne Kemmling, Goettingen, Germany.)

Figure 2. Infection cycle of influenza viruses. Following uncoating, the (–)RNA segments (blue) travel into the nucleus where they are replicated. The (+) strands leave the nucleus and, by translation, the viral proteins are synthesized. Neuraminidase and hemagglutinin molecules are incorporated into the membrane. Capsid proteins travel into the nucleus, where the virus particles (without the membrane) are then completed. (Drawing: Anne Kemmling, Goettingen, Germany.)

Figure 3. Infection cycle of HIV. Virus particles bind to CD ₄ receptors of immune T cells. Following fusion with the cell membrane, particles are released and the two RNA molecules are uncoated. As described in the text, each RNA strand is transcribed into a DNA strand and further into a piece of double-stranded DNA, which integrates into a chromosome. Transcription yields (+)RNA molecules, which serve as matrix for the viral proteins as well as genetic material for progeny. (Drawing: Anne Kemmling, Goettingen, Germany.)