Infectious diseases are caused by microorganisms such as viruses, bacteria, fungi or parasites. Microorganisms that cause disease are collectively called pathogens. Pathogens cause disease either by disrupting the bodies normal processes and/or stimulating the immune system to produce a defensive response, resulting in high fever, inflammation and other symptoms. Infectious diseases can be spread from one person to another, for example through contact with bodily fluids, by aerosols (through coughing and sneezing), or via a vector, for example a mosquito. Mosquito, Close up of a mosquito feeding on human blood. Infectious diseases are one of the leading causes of death worldwide. Many diseases become difficult to control if the infectious agents evolve resistance to commonly used drugs: For example, bacteria can accumulate mutations in their DNA? or acquire new genes? that allow them to survive contact with antibiotic drugs that would normally kill them. Scientists are currently searching for new approaches to treat infectious diseases, focusing on exactly how the pathogens change and drug resistance evolves.