(A) 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 nearly eradicated by widespread vaccination. (B) The bacterium Vibrio cholerae, the causative agent of the epidemic, diarrheal disease cholera. (C) The protozoan parasite Toxoplasma gondii. This organism is normally a parasite of cats, but it can cause serious infections in the muscles and brains of immunocompromised people with AIDS. (D) This clump of Ascaris nematodes was removed from the obstructed intestine of a two-year-old boy. (A, courtesy of Robert Grant, Stephan Crainic, and James M. Hogle; B, all attempts have been made to contact the copyright holder and we would be pleased to hear from them; C, courtesy of John Boothroyd and David Ferguson; D, from J.K. Baird et al., Amer. J. Trop. Med. Hyg. 35:314–318, 1986. Photograph by Daniel H. Connor.)