Every epidemic, has got a specific course of a progress along with time, which the epidemiologists diagramatically represent in a bell shaped curve, Epidemic Curve. It has an ascending limb, when number of cases gradually increase, then a plateau, when number of cases stabilizes a level, which may be flictuating little bit, but representable more or less by a rough straight line, and following a descend line, where the number of cases (or, deaths) starts reducing gradually. This is because, the people incurring immaturity, may be due to asymptomatic, (and) mild to moderate infection, not resulting in death. Mean while, the virus may get less virulent and may lost its infectivity.
On the other hand, when a rather new virus is introduced in a community, who have no immunity against the virus infection, maximum number of people willl be affected, as they have got no acquired or natural immunity, against the virus. This ranges of infection may be very serious leading to severe cases leading to death, or may be moderate to mild cases not suffering very much or asymptomatic cases, not detectable by usual method, but only by laboratory tests. But gradually, with passage of time, majority of the people in the community, this by acquiring immunity. Thus at last a stage will come, when the non immune but susceptible cases will be distributed far and wide, so that person to person will become impractical practically. This phenomenon is termed by the epidemiologist herd immunity.
With reference to COVID 19, the different countries are at different position on the epidemic curve. China might have reached the plateau on the epidemic curve, but other countries, which are reporting late may be in the ascending limb. As COVID is primarily a person to person spread disease, the shape of the epidemic curve will have a long protracted tail.
For COVID 19, our experience is really novel one, and we need to study its different parameters thoroughly and critically to understand the dynamics, which definitely will take some time
HIFA profile: Tusharkanti Dey is a Community Health Specialist at the Center for Total Development in India. Professional interests: Developing community health projects based on ICT. dr.tusharkantidey AT gmail.com