Acknowledgements Thanks to the following for permission to reproduce photos: Cover Colin Varndell/SPL; Fig 1.4 STEVE GSCHMEISSNER/Getty Images; Fig 3.7 LAGUNA DESIGN/SPL; Fig 5.4 STEVE GSCHMEISSNER/SPL; Fig 5.5a SCIENCE PICTURES LIMITED/SPL; Fig 5.5b SCIENCE PHOTO LIBRARY; Fig 5.6 ARTURO LONDONO, ISM/SPL; Fig 7.4 Ed Reschke/Getty Images; Fig 7.7, 7.11, 7.13, 14.5, P1 Q4 © John Adds; Fig 7.9 DR KEITH WHEELER/SPL; Fig 8.1a-d Dr. Gladden Willis/Getty Images; Fig 9.4, 9.5, 11.1, 13.16 BIOPHOTO ASSOCIATES/SPL; Fig 11.4 Cheryl Tyron/CDC/Getty Images; Fig 11.5 DR M.A. ANSARY/SPL; Fig 12.6 Bengal at a glance/Getty Images; Fig 13.10 JEFF CARROLL/AGSTOCKUSA/SPL; Fig 14.4 DR P. MARAZZI/SPL; Fig 15.6 Panel Rey/EyeEm/Getty Images; Fig 16.1a, b Alila Medical Media/Shutterstock; Fig 16.8 SCIEPRO/Getty Images; Fig 16.9 SIA KAMBOU/AFP/Getty Images; Fig 17.3 Axel Bueckert/Getty Images; Fig 17.6 JEFF LEPORE/SPL; Fig 17.8 PAUL D STEWART/SPL; Fig 17.9 Science VU/Getty Images; Fig 17.12a Watcha/Getty Images; Fig 17.12b randimal/Getty Images; Fig 18.1 MICHAEL MARTEN/SPL; Fig 18.2 ephotocorp/Alamy Stock Photo; Fig 18.4 JOHN SHAW/SPL; Fig 18.8 EYE OF SCIENCE/SPL; Fig 18.9 LOUISE MURRAY/SPL; Fig 18.10 RICHARD R. HANSEN/SPL; Fig 18.11 M.H. SHARP/ SPL; Fig 18.12, 18.13 FRANS LANTING, MINT IMAGES/SPL; Fig 18.14 imageBROKER/Alamy Stock Photo; P2.6 Nigel Cattlin/Alamy Stock Photo; P2.8 Jennifer Booher/Alamy Stock Photo SPL = Science Photo Library
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