Glossary terms with N A B C D E F G H I J K L M N O P Q R S T U V W X Y Z N Natural compound library A natural compound library is a collection of chemical substances, which can be used to screen for biological activities of choice. Natural compounds A natural compound is a substance produced by bacteria, animals, fungi or plants that fulfills a biological function. Natural killer cells Natural killer cells are lymphocytes belonging to the innate part of the immune system, that directly kill virus infected cells and tumor cells. Neglected Tropical Diseases Neglected Tropical Diseases are a group of communicable diseases that are mainly found in tropical and subtropical countries and that mainly affect people living in poverty. Nucleocapsid A nucleocapsid is a viral structure unit composed of nucleic acids and attached proteins.