To lower the risk of contracting infectious disease, it pays to understand the concept of “immunity”. Immunity refers to the ability of an individual to resist a particular infection or toxin by the action of its immune system. This can refer to resistance of an entire species (humans, for example, don’t get fin rot) or the resistance of a particular individual to an illness (Typhoid Mary, for example).
The level of immunity to disease often correlates with the general health of an individual or even a species. Human populations under stress, as in wartime or in survival circumstances, may have weakened immune systems and be less resistant to an infection. This resistance level isn’t always stable; it can change markedly due to a number of factors. These include:
- Age: The very young and the elderly are, typically, the most susceptible to infection.
- Diet: Malnourishment depresses the immune system.
- Environment: hot/cold/wet/dry conditions contribute to weakening the immune system of the unprepared.
- Stress: chronic stress can wear down an individual’s ability to fight infection.
- Injury: The energy required to heal an injury limits the ability to resist infections.
- Fatigue: Anyone who is overworked or sleep deprived will find their immune system weakened as a result.
- General health status
- Presence of other diseases: Diabetes and other illnesses notoriously affect an individual’s ability to resist infection
- Length or quantity of exposure to a disease-causing organism ( otherwise known as “pathogens”): Even healthy individuals may be exposed so frequently or to such a large quantity of pathogens that they succumb to infection.
There are, from a practical standpoint, three separate levels of immunity. They are:
1. SHORT-TERM IMMUNITY
When an infectious agent is detected, the body responds by producing a large amount of white blood cells (known as “leukocytes”), an immune response which attacks the invader. If successful in destroying the pathogen, the white cell count returns to its original level. White cell counts are part of a complete blood count, a common test performed in patient lab panels. During an epidemic, the human population’s ability to generate white cells increases its resistance. It is this property of the human body that causes the epidemic to eventually collapse.
2. LONG-TERM IMMUNITY
The body’s defenses actually retain a type of ‘memory’ of the offending organism. If the pathogen returns to the area, that memory causes the body to produce a faster and stronger response against it. This is especially true with viral infection, often giving lifetime protection. An example would be “Varicella”, a viral illness otherwise known as ‘chicken pox”. Once you have had chicken pox, you are usually immune for the remainder of your life. The same cannot be said for viruses that frequently mutate, such as Influenza. In bacterial infections, the body’s memory may decrease over time, and re-infection is not uncommon.
3. NATURAL IMMUNITY
A particular individual, or occasionally an entire species, might possess the ability to resist a pathogen due to genetic “memory” passed on from generation to generation. The advantage could exist, for instance, as a certain blood type which hinders nourishment of the organism.
When a community experiences its first outbreak of a disease, deaths will occur among the susceptible. The Native American population of the New World had an extraordinarily high mortality rate when exposed to smallpox by the first European explorers. Those same explorers, however, had a much higher survival rate due to genetic advantages given by centuries of previous exposures.
Over time, however, the survivors reproduced and more of the community was able to survive re-exposures to the infection. Thanks to the passage of a new “memory”, their descendants achieved a much higher natural resistance level. As a result, epidemics of the same infection became less deadly and, in some cases, disappeared.
4. HERD IMMUNITY
When a large group (a “herd”) possesses immunity, susceptible individuals enjoy a certain protection due to fewer exposures to an infection that may otherwise be fatal. The most common example today relates to vaccinated populations. If an un-vaccinated person moves into an area where many are immune, the likelihood of exposure drops significantly. This confers a certain level of protection; there is less chance of that infection affecting the community that person is currently in. If many un-vaccinated people move into an area, however, the overall “herd immunity” may be lost.
Joe Alton, M.D., aka Dr. Bones