Understanding epidemic, pandemic, and endemic

An epidemic is an outbreak of disease that attacks many people at about the same time, over a short time period in a community, and may spread through several communities in adjacent areas. Examples of this is the cholera epidemics that recur in parts of Africa and south Asia, the Ebola epidemic, SARS, and the flu epidemics that happen in the Americas at certain times of the year. When an epidemic spreads over a larger area and jumps from one country to another, infects people in many countries and crosses continents, then it becomes a pandemic. Examples of these were the Black Plague and the Spanish Flu which infected almost all Europe. Due to the globalization with the advances in easier and faster travel and transportation, which allow more people to move all over the world, Pandemics will occur more often, unless mitigating and advances in medical and healthcare are able to control the spread and the disease itself.

Endemics are diseases that exist permanently in a particular region or in a population. Malaria, as an example, has largely been endemic in underdeveloped countries that hosts certain type of mosquitoes, and polio is endemic in places that have not allowed vaccination for social or religious reasons. These diseases don’t infect many people because most of the population has developed antibodies against the disease when they were earlier infected or they were vaccinated against the disease which generated antibodies. Endemics, therefore, don’t infect as many people, and if they do infect it is over a longer period of time, so it doesn’t overwhelm hospitals or healthcare facilities. This is the “herd immunity” concept which posits that when enough of the population is infected, then many of them will have developed the antibodies that they will not become gravely ill or die. The death rate will then become tolerable like any other illness.

This “endemic” concept is one of the reasons some leaders of countries and some US governors didn’t implement early isolation and social distancing, quarantine, and lockdown in their territories despite the COVID-19 pandemic. The other reasons are the economic impact of the restrictions and the perceived low rate of infections in their areas. Now, that we are in the fourth month of this COVID-19, the herd immunity argument is less talked about as the “exponential” behavior of the COVID-19 infection and death rates are devastating all countries.

Looking at the COVID-19 infection and death rate charts (actual and projected) worldwide and in specific countries, it is always “bell shape” plotted with time on the horizontal axis and infected population in the vertical axis. The taller/higher the apex of the curve, the deadlier is this pandemic, so the objective is to “flatten the curve” and this can only be flattened by isolation, social distancing, quarantine, and lockdowns. Ideally massive testing should have been done so we know who to isolate and where quarantining and lockdown will be most effective. But as very few countries were able to do massive testing, we have to include everyone in these mitigation measures, and suffer the economic consequences.

I understand “exponential functions” having 18 units of college and graduate mathematics but not enough calculus, so I can appreciate the danger of the exponential infection and death rates of this COVID-19 pandemic. The “inflection points” in many of the model curves have been calculated in some countries and it shows that it could be in May and June, so the curve should be coming down in July. If this will happen, then the exponential curve becomes a “Poisson Distribution Curve” and if the curve is flattened it becomes a “Normal Curve”. Then COVID-19 will be an endemic next year.