in turn can be fed into a mathematical model for the epidemic. Based on the reproduction number, we can define the epidemic waves, such as the first wave, second wave, etc. for COVID-19 in different regions and countries. Misuse While it might be common to say that AIDS is endemic in some countries, meaning that it is regularly found in an area, this is a use of the word in its etymological, rather than epidemiological or ecological, form. Some in the public wrongly assume that endemic COVID-19 means the disease would necessarily be mild. Endemic COVID-19 could be mild if previously acquired immunity reduces the risk of death and disability during future infections, but in itself endemicity only means that there will be a steady, predictable number of sick people. Related terms Categories of endemic diseases Holoendemic An endemic disease with an extremely high rate of infection, especially a disease that infects nearly everyone early in life, so that nearly all adults have developed some level of immunity. Hyperendemic An endemic disease with a high rate of infection, especially one affecting people of all ages equally. Mesoendemic An endemic disease with a moderate rate of infection. This term is often used to describe the prevalence of malaria in a local area, with 10 to 50% of children showing evidence of prior infection being considered a moderate level for that disease. Hypoendemic An endemic disease with a low rate of infection. Typhoid fever is a hypoendemic disease in the US. Categories for non-endemic diseases Sporadic A disease that appears occasionally, but, unlike endemic disease, is not always present at a steady and predictable level. Epidemic A new disease that is spreading or a previously endemic disease whose infection rate is increasing significantly. Seasonal flu frequently appears as an epidemic. Outbreak An epidemic, especially one affecting a very small area, such as the people in one town or attending a single event. The 2019–2020 measles outbreaks showed a normally endemic disease causing an epidemic outbreak, primarily among unvaccinated people. Pandemic An epidemic affecting a very large part of the world, generally multiple countries or multiple continents. Seasonal flu is sometimes a global pandemic. Examples This is a short, incomplete list of some infections that are usually considered endemic: Common cold Lassa fever Malaria Polio Rotavirus Hepatitis C Measles Smallpox was an endemic disease until it was eradicated through vaccination. Etymology The word