AstraZeneca ( AZN) says it's investing $100 million to boost its work in developing infectious disease and cancer treatments. As a part of its plan, the pharmaceuticals company will add 100 researchers to its team of 400 existing employees who are focused on those diseases. "Infectious disease is becoming more prevalent and problematic to patients and healthcare providers, placing a heavy burden on our health care systems," said Jan Lundberg, head of research at AstraZeneca. "With our expanded presence in Boston, our scientists will be better equipped to discover and develop novel medicines to address important areas of unmet need." Shares fell 31 cents to $56.44 early Monday.