The immune system's capability to fend off pathogens is due to its adaptability in creating specialized defense mechanisms. When the immune system is exposed to a pathogen for the first time, cells called dendrites break it down into fragments called antigens. These antigens are taken to a lymph node or the spleen - the “headquarters” of the immune system - where they are used to activate cells specifically designed to attack the pathogens: the lymphocytes. Every type of lymphocyte is antigen-specific; they are only activated by their matching antigens. Once activated, lymphocytes begin to multiply rapidly and either turn into effector cells (the “frontliners” that fight the pathogens) or memory cells (the “backups” that stay in the background). While the effector cells attack the invaders and die within weeks, memory cells remain in our bodies where they can be activated in the future in much the same way as their parent lymphocytes.
The downside to this strategy is that it is slow. One type of lymphocyte can be as rare as one in 100,000 and must arrive at the lymphoid organ where its antigen is present, or encounter its antigen while in circulation. The lymphocyte must then replicate quickly to match the pathogen's exponentially growing population. During this process, the pathogen might have already done considerable damage to our body.
However, when numerous lymphocytes of the same type are in circulation, the activation time can be much shorter, allowing them to possibly defeat the pathogen before it causes harm. But such legions of lymphocytes are only created during the first exposure when the parent lymphocytes have multiplied into memory cells. Thus, a person must first be exposed to the pathogen - and survive its affliction - before becoming immune. After that, future battles are rigged against the pathogen; oftentimes the pathogen is defeated without the human ever recognizing that they had been infected.
Unfortunately, some first infections proved fatal. The Jarawas, an indigenous tribe in the Andaman Islands, were believed to have been decimated by the diseases brought by the British in 1789. Only a small percentage of the population survived. Another indigenous tribe in the archipelago, the Jangils, was suspected to have gone extinct for the same reason. Around this time, the British also visited the North Sentinel, where they kidnapped two adults and four children and brought them to Great Andaman Island. All of them became incredibly sick. After the two adults died, the British returned the sick children to the island, where they had likely spread the disease among other tribesmen.
Foreign pathogens spread by the British had also been a problem in other places. During the American Revolutionary War, more soldiers died from diseases than weapons of war. The Americans were at a disadvantage during the early months of the war as they battled against another enemy aside from the British: the variola virus, a pathogen that causes smallpox. The British troops, consisting of soldiers that had already been infected by and survived smallpox when they were still boys, did not have to worry about this. They gained immunity early due to the virus first spreading in Europe before North America. Left behind, most American soldiers had not yet caught the virus when the war broke out.
All of this changed when the Americans realized they could train their immune system by exposing themselves to a weaker “cousin” of the virus that causes much milder, less life-threatening symptoms. This created immunity against it and its close relatives, including the variola virus, thereby becoming immune to smallpox without having to suffer its tyranny. This process is known as inoculation, now more commonly referred to as vaccination. Today, most vaccines contain pathogens that are either killed, weakened, fragmented, or modified, and they act in the same way as the pathogen's weaker cousin.
To Chau's credit, he received 13 immunizations before traveling to North Sentinel and had quarantined himself by living in the Andaman Islands for a month. But the microorganisms he brought with him still posed a grave threat to the Sentinelese. Chau carried with him a vastly different set of microorganisms compared to what the Sentinelese have, potentially including viruses to which no vaccine is available, such as those that cause common colds.
When babies are born, they inherit microbes from their mothers. The microbes in the gut of the mother are transferred to that of the baby, and microbes from the mother's vagina become the first microbes that inhabit the baby's skin. When the baby is delivered through cesarean, they acquire their first skin microbes from the environment they are first exposed to. These microbes, called the microbiome, become a part of the human as much as their cells. And just like the cells, the microbiome is distinct for every person. The microbiome evolves with the person, and the immune system develops a delicate relationship with it.
Among several benefits of the microbiome is its role in training and helping the immune system. Living with the microbiome, the immune system has evolved to create a slippery boundary between the cells and the microbiome, called the mucosal firewall. This firewall keeps pathogens from easily infecting cells. Moreover, the microbiome occupies space and consumes resources that would otherwise be for pathogens, further adding protection. Lastly, particular microbes in the gut can activate some types of lymphocytes the same way as antigens, but much faster. This helps the immune system to easily catch up with the invading pathogen.
But not all members of the microbiome are helpful. Some have the potential to cause diseases, called photobionts. Luckily, our immune system has learned to subvert these photobionts such that we can live with them safely.
A person's microbiome and immune system balance each other out into a delicate, beneficial relationship. But when this relationship is disrupted, it can cause unwarranted diseases. One cause that can tip the balance is a sudden change in the microbiome, called dysbiosis. The dysbiotic microbiome deregulates the immune system by signaling it to falsely activate or lowering chemicals that stabilize it. In most cases, this results in inflammation.
Inflammation and common colds might not be too concerning, but this weakens the immune system, which further disrupts the delicate immune system-microbiome relationship. When this happens, the immune system will fail to protect the body from the photobionts, and the microbiome that was once harmless will turn against the human. A simple inflammation or common cold can cascade into more serious, more life-threatening diseases.
Chau would have caused dysbiosis among the Sentinelese and would have infected them the same way as the British. Worse, we have no way of helping them; doctors cannot simply visit them, and they do not understand other languages, even the local Andamanese. For the Sentinelese, their hostility against Chau was a matter of self-defense.
On November 17, after numerous stubborn attempts, Chau again visited the island, this time for good. He had instructed the fishermen to leave without him. But shortly after dropping him off, they saw him getting dragged by the tribesmen across the beach. They came back later that night and saw Chau's lifeless body on the shore.
Chau wasn't oblivious. No, Chau knew exactly what he was going into. He prepared for the mission for months. He focused on his physical health. He quarantined for a month. He received vaccinations. He even learned new languages. Above all, he accepted the fact that he might die.
After failing to retrieve Chau's body, the authorities abandoned future efforts to do so. It was deemed too dangerous, both for them and the Sentinelese. Chau was aware of the risks. In his last journal entry, he wrote an underlined note in the margin that said, "Don't retrieve my body."