I would like to thank many individuals for their generous help and support to complete this thesis.

I am deeply indebted to Dr. Gabriele Schönian (Dept. of Microbiology and Hygiene, Humboldt University) for her continued valuable suggestions, useful scientific comments, enthusiasm, guidance, constructive criticism throughout this work. Indeed thank you very much for your unflagging efforts in making it possible to defend this thesis at the Humboldt University (Berlin, Germany). Really it has been a privilege and honours to work with you both as a scientist and a person.

I am also most grateful to Professor Wolfgang Presber (Dept. of Microbiology and Hygiene, Humboldt University) for his valuable comments, support and encouragement. Many thanks to Professor Richard Liucius (Faculty of Biology, Humboldt University) for his supervision

I also acknowledge Dr. M. El fari for his general help and his wife Fayza who really made me feel at home. Also I would like to express my thanks to Dr. Wanda Sokdowska-Kohler for her help during my stay at the Institut für Mikrobiologie und Hygiene (Berlin, Germany). My thanks also extend to the technical staff at the same institute for their excellent help during the practical work.

Also I would like to thank Mr. Hamed El Neel and his technical staff (Gedaref Clinic, Sudan) for collection of clinical samples and to Khalid O. El Tai (Faculty of Agriculture, University of Khartoum) for his help during the fieldwork.

I am also grateful to Professor Michael A. Miles and to Dr. Isabel Mauricio for their assistance, provision of useful comments, and for the experience I gained during my stay at London School of Hygiene and Tropical Medicine, Dept. of Infectious and Tropical Diseases, Pathogen Molecular Biology and Biochemistry Unit.

Also I would like to thank Zuhier M. Elzubier for his assistance in the layout of the illustrations.

I also gratefully thank my husband for his encouragement, support, help and his tolerance of the prolonged disruption of normal social life.

Last but not least, I would like to thank my parents for their meticulous moral support and encouragement from primary education until I finished my Ph. D.

This work is generously financed by German Academic Exchange Service (DAAD), WHO (TDR Research Training Grant) and a grant from the Institut für Mikrobiologie und Hygiene, Faculty of Medicine, Humboldt University, Berlin.