Why don’t chronic diseases kill us?


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EPIDEMICS and PANDEMICS are two categories of diseases with respect to the spread of the disease – An epidemic is the spread of an infectious disease to a large group of population, of a region, within a short period of time, while a pandemic is an epidemic which has escalated worldwide. Every few years, the emergence of an infectious disease causes epidemics and pandemics. The most popular one is of AIDS, standing for Acquired Immunodeficiency Disorder, which was first considered an epidemic, then further became a pandemic.

Relevant diseases which exhibit pandemic properties are Swine Flu and Corona Virus. While extreme measures are taken to keep these diseases at bay, it has observed that people suffering from these disease exhibit chronic symptoms, but fail to be fatal for the individual. For example, the MORTALITY RATE (i.e., the number of deaths from a disease in a particular region) of the pandemic Corona Virus is merely 3.4%, which is a small fraction of people, leaving the larger portion of patients suffering from the symptoms but not dying because of them.


This raises the question of why Corona virus is a major concern to the world when it is not even proving to be fatal to a big fraction of the population. It also raises a subsidiary question of why the Corona virus is not killing people, inspite of it being a chronic and serious disease.

Does a pathogen killing its host harm itself?

A hypothesis put forward by scientists is of that – a pathogen killing its host is harming itself. Since a host (in case of Corona virus, humans and bats, to name a few) provides the pathogen (the virus) with nutrients required for its growth and propagation, it would be unreasonable for the pathogen to kill the host and lose out on its chances of survival.

That being said, Corona virus does have a mortality rate of 3.4%, which means that the virus does kill it hosts in certain cases – this is during the infectious stage (the stage of the disease wherein the symptoms are extreme and the propagation of the virus is at its optimum), which gives the virus the opportunity to spread and propagate in others. Tapeworms are considered the “perfect parasites” since they show no obvious symptoms in the patients, thus ensuring that the hosts can spread the disease by shedding tapeworms eggs, which will be ingested by others, thereby infecting them as well and increasing its chances of survival.

corona outside china

Does the pathogen itself not kill the host?

The other explanation of the phenomenon of a few fatalities is the occurrence of a SECONDARY INFECTION. A secondary infection is one which attacks the body when the immune system of the body has been weakened by the first infectious agent. This can be understood by taking the example of HIV (Human Immunodeficiency Virus). HIV is a sexually transmitted disease (STD) which has become a pandemic over the last decade. It is a “popular” disease in the scientific community since it is known to be incurable and known to not kill the host.


A person with HIV can survive years without exhibiting any symptoms – the symptoms are presented only with a drastic decrease in the immunity of the body. Thus, it paves the way for other pathogens, such as those causing pneumonia, to attack the body and infect it. Due to this decreased immunity, the secondary infection is the one which kills the host, rather than the HIV itself. This mechanism can be true for other infections as well.

Have pathogens evolved in a way to not kill their host?

It seems reasonable to assume that some pathogens have evolved in a manner so as to survive by multiplying in their host and not killing them. Such a relationship is called COMMENSALISM (an association between two organisms wherein one is benefited, while the other is unaffected. In this case, the pathogen is benefited, while the host is not affected until the onset of the symptoms).

This theory stems from the fact that evolution helps organisms who can adapt to their environment to survive. If the pathogen were to COEVOLVE (hand-in-hand evolution of two organisms, wherein the behaviour of one greatly affects the other) with their hosts, their survival rates would greatly improve – thus, it is safe to say that viruses such as Corona virus and HIV have evolved with their hosts and changed their mechanisms to not lethally harm the patients.

When we take into consideration the occurrences of diseases such as tuberculosis and typhoid, which are relatively common and chronic in nature, logically, the diseases should kill us. But with advancements in medicine and the inherent property of pathogens to survive and reproduce, the mortality rates of chronic infectious diseases are low. While months of medications and treatments are required to overcome the diseases, the chances of someone dying from them are low – but the likelihood of them spreading is the “endgame strategy” of the pathogens. So, it is extremely important to maintain proper hygiene and be aware of the source, chain of transmission and symptoms of the diseases and consult a medical professional for the same.

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