Since its emergence in 2012, NIAID scientists have advanced their understating of how MERS-CoV causes disease, focusing on developing animal models of disease and countermeasures to disease. Work has shown that for MERS-CoV to infect a person, the virus enters cells using the spike, or S protein. After entering the cell, the virus delays the normal immune system response, allowing the infection to gain a foothold in the body. By the time the immune system recovers, the infection has progressed and becomes much harder to fight. Scientists also have characterized different strains of MERS-CoV and can determine through tests how those stains affect different animals and how their form—liquid or aerosol—affects their stability. A rhesus macaque model of MERS-CoV has shown that clinical signs of disease appear within 24 hours of infection, and the virus causes disease deep within the lungs, leading to pneumonia. Researchers use the rhesus and other models to study how MERS-CoV causes disease and to evaluate potential vaccines and treatments. NIAID-funded researchers also have established several small-animal models of infection in mice that have been critical in the advancing the development of MERS-CoV medical countermeasures.