Before any individual can be infected, there must be a point of entry. For us humans, most of our sense organs are point of entry for microorganisms, and they also double as initiation point for infection most times. Hence, we have the following entry points for infective microorganisms: mouth, ear, nose, eyes, skin and sex organs (genitalia).
Of course, only specific microbes can cause infections in these areas, with some of them being localised or disseminated as shown in this article. Also, among the many factors that determine if infection can occur in anyone, three of them are prominent:
- The specificity of the infectious agents for the target organ/tissue in that area: We have to understand that not all microbes can cause infection in the localised organs that they enter. For instance, organism like Salmonella typhi that causes typhoid fever cannot cause infection in the genital areas. That organism is known to cause gastroenteritis, and not STIs. Also, the notorious organism causing candidiasis in most female genitals, Candida albican, cannot cause any infection in the respiratory tract of humans. This is usually the modus operandi. Except an organism is capable of multiorgan infection, which is prominent in some viruses like Ebola virus, or there is a disseminated infection which will be made complicated by our own immune responses, most organisms have their target organs and will not affect other organs.
- The load of the infectious agent: This second point has to do with the actual number of the microorganism capable of overpowering both our innate and adaptive immunities. Let’s use a practical example. Escherichia coli is usually the indicative organism used to determine the purity of water in the laboratory in terms of drinking, and there is an international standard for the number of the organism that should be found in any water for it to be considered drinkable or not. Above this threshold value, infection is highly likely to be initiated. Let’s say if 20 and above microbes should cause an infection, and 10 of those microbes are the ones able to get to the target organs, infection might not start provided the third factor discussed below, the competence of the immunity of the person is high.
- The immunocompetence of the infected individual – This has to do with the capability of one’s immune system to fight infection at any given time, and unfortunately, it varies day to day. A person whose immune system is incapable of fighting infection at a specific time is called an immunocompromised person, and several factors can make this happen, depending on the site of entry of the infected individual. We will discuss more about these factors as we go on because they are intrinsically linked with the topic of discussion.
We will have to break this post into parts in order to avoid it being too long and boring. But before we go, I will like to quickly point something out and differentiate between infection and diseases. Infection is the presence of an organism capable of causing diseases in our body system, while a disease is the manifestation of the infection after the infecting organism might have overpower the immune system and started showing symptoms. So, the truth is that most of the time, we are always – I mean always – infected with one disease-causing agent or the other. Why we don’t usually have the symptomatic manifestation called disease is mainly due to those factors stated above among others.