At what point a virus spread is declared as community transmission?

Maintaining the social - distance is relevant under the present circumstances to avoid the contagious effect of this virus and as such a distance of minimum of one meter should be maintained at all levels apart from wearing a suitable mask covering both nose and mouth to arrest the transmission of the COVID 19.
Community transmission of this disease is possible due to the following factors -
1) The droplets of virus is suspended in the air and a person having a low immunity comes in contact with such droplets.
2) Progression of the disease is easier in case of non maintenance of basic norms such as social distancing and not adhering to mask wearing.
3) Concentration of the virus germs is free to flow from one region to another due to high volumes of the suspended droplets of the virus due to high population affected with this deadly virus.

For an infection spread there is first primary source. The Covid19 virus did not originate from India. Infected persons came to India from another country(s).That person(s) is the primary source.
Secondary Source or primary Contact infection: The primary source persons (s) come into contact with other persons like family members, other persons while shopping, travelling visiting worship places etc. This is primary contact infection or secondary source.
These secondary source infected people also come in contact with many other persons. Some of them may become infected.Usually the testing is done for primary sources and known primary contact sources by tracing and mapping contact history.
However as there is some gestation period for infection to develop and symptoms to be exhibited, within the gestation time one or more asymptomatic infected persons would have come in contact with many persons. As now asymptomatic person are not tested (unless it is known that they have come n contact with suspected infected person), many asymptomatic persons become carriers of the virus and might have infected many others.
It is at this juncture that many people become infected and do not know the source from where they got infected. Man such infected persons would have taken precautions generally also and avoided any physical contact with any known infected person. At this juncture, the rate of new positive results (newly reported cases) grows in high rate every day and rate doubles within a short period. The rate of number of recovered cases is far below the reported new cases. This is the stage of community spread.
Community spread is when the infected person does not know the source from where he got the infection as he had not come in contact with any symptomatic source or primary or secondary contact of primary source. Infection can spread from any one to any other person.
Community spread occurs in crowded clusters, in closely populated colonies; when people move around without following restrictions like mask and social distancing and when not following the hygienic instructions. People coming in contact in congregations, enclosed spaces, crowded places, sitting in air conditioned rooms with others even if not crowded, are prone to get infected if infected asymptomatic or symptomatic people are/were there in their proximity or contact.

When a deadly virus spreads its infection across the people in any area then it does it in three stages.

The first stage is the first case which is generally caught from the first carrier who had come to that place from the outside where that virus has already spread considerably. So, in the first case the carrier of the virus is known and then the person effected has to be admitted in the hospital and his treatment is to be started. If these two people are well isolated and none gets infection from them then also there is no problem. The outbreak would be nipped in the bud in that place.

If opposite happens (which usually happens) that some people get infections from these two and then it start multiplying afterwards and that leads to second stage. But still in such cases you can track and trace the source of each and every person and then take the requisite steps in isolating them. This second stage can be managed well but eventually it only is going to transform in the third stage. It is easy to make containment in the second stage if we have good infrastructure.

Now the third stage as mentioned above is the stage of community level in which the mixing is so much that we cannot identify and isolate the person or object from which someone caught it. This is the community level spread that makes the life of doctors and scientist difficult as they do not get any additional information and the police and local administration is also at their wits end to tackle these situations. As the source of the infection cannot be ascertained in the community level infection, there is no way to stop it or contain it. Sometimes it increases in exponential proportions also and becomes almost impossible to contain or control.

The experts say that sometimes during the community spread the herd resistance or herd immunity is created in the human beings as the reaction of body for these outside attacks and once that is created the virus attack slows down gradually.

COVID-19 has three stages. As everybody knows COVID-19 came from China. So the carrier of the virus came from foreign countries. When the source of an infected person is known then the disease is called to be in the primary stage. In this stage, the patient can be isolated and treated easily. When people get an infection due to come in contact with an infected person like a family member and friends and colleague then the disease is said to be in the second stage. Infection is said to be in the third stage when the source is not known. In this stage, it is difficult to isolate and identify the source of an infected person from where got an infection. It is called a community-level transmission. At present, whether India is in the second stage or third stage would be difficult to say.
We should maintain social distance and wearing a mask to stop spreading this disease.

Now corona spread every country in the world.
It has three stages given by the WHO -

First Stage - People who came from affected countries could be carriers of the virus so in this stage tested every person who came from another effected country.

Second Stage - People may have been in contact to the person who came from another country which country is affected from the virus so in this stage test would be done for all those people who came to the contact, in this case carrier can be known.This stage called Local Transmission.

Third Stage - When cluster of cases are detected and do not getting information about carrier this conditions is very serious and called community transmission.In this stage it is most chances to impossible to break the chain of virus transmission as people have no idea that how did they infected.

So for now we just need to save us and our family be alert and follow the guidelines given by the WHO -
Social distancing
Keep washing hands
Always use mask