Mononucleosis, commonly called mono, is an illness caused by the Epstein-Barr virus. Although it's rarely serious, mono can cause bothersome symptoms that last several weeks.
Complex systems. Complex systems present problems both in mathematical modelling and philosophical foundations. The study of complex systems is an approach to science that investigates how relationships between parts give rise to the collective behaviors of a system and how the system interacts and forms relationships with its environment.
The first question is easy to answer: ecology is the study of interactions between organisms and their environments. Interactions is the key word here. Ecologists do not only study organisms; they also study how organisms interact with other organisms and how they interact with the nonliving parts of their environments, like chemicals, nutrients, habitats, and so on. As you might suspect, ecology is not as simple as it sounds.