Transmission of an infectious disease can occur via diverse pathways as each infectious agent has evolved to exploit a particular organism or cell type. Most microorganisms do not cause disease. In order to indicate that a disease is infectious, it must be demonstrated that the disease-causing pathogen is present in ill people but not in healthy people and that healthy people who contract the pathogen develop disease. The severity of the disease depends on the ability of the pathogen to damage the host as well as the ability of the host to resist the pathogen.
Respiratory diseases such as influenza are contracted by exposure to aerosolized droplets spread by sneezing and coughing. Gastrointestinal diseases, many of which cause diarrhea, are contracted by exposure to contaminated food or water, while sexually transmitted diseases are acquired through contact with body fluids. Some pathogens can persist on an inanimate object (common cold) and others can be transmitted via skin to skin contact (warts). Malaria, Lyme disease, West Nile, and Chagas disease are transmitted by insect vectors. Diagnostic methods used to identify disease include clinical presentation (ex. warts), microscopy, biochemical test (ex. ELISA), growth culture, and molecular diagnostics.
Epidemiologists are researchers interested in how an disease is transmitted, the source of the disease, numbers of people infected, geographical distribution, risk factors, and mortality rates. Epidemiologists also study chronic disease, such as heart disease and diabetes, illness due to chemicals and other pollutants, genetic disease, injuries, mental illness, and the risks and benefits of drugs. Epidemiology relies on biology, medicine, and biostatistics in its evidence-based approach. The American Red Cross tests all donated blood for Chagas disease, Hepatitis B, Hepatitis C, HIV, HTLV, syphilis, and West Nile virus.