Host Susceptibility and Host Barriers

00:00 So, on the other side, so we've been talking about the virulence of microbes and their cleverness. Let's talk about the host and not all hosts are obviously created equal, there are issues relating to host susceptibility that will, in many ways, determine whether a microbe will be successful on its attack. So, age or developmental status, the very young and the very old tend not to have mature immune responses and therefore will be relatively immunocompromised. There is nutritional status. So if you are cachectic starving, you are not making the normal amounts of inflammatory cells, you're not making the normal amounts of inflammatory mediators, the liver is not synthesizing important complement things, like that. So nutritional status is clearly very important. If you have concurrent disease. So if you have an ongoing, say autoimmune disease that is affecting your immune status, that can potentially impact the host susceptibility. Prior or concurrent infections are a very important point. So, in patients, for example who have influenza that is a lytic infection of the airway epithelium. If I'm getting a lytic infection, now I am making that airway and the lungs distal to that airway more susceptible to a secondary bacterial infection. And in fact, in the Spanish flu epidemic back in 1918, many of the people died not of the primary influenza but of the secondary bacterial pneumonia that happened because they had denuded their airway epithelium and could no longer clear bacteria effectively. Barrier status is really important. So if you have had a major burn over a certain part of your body, you've lost that skin barrier and you're now going to be more susceptible to infections.

02:00 So that's kind of a no brainer there. The immune status is also really important. An immune status is impacted by age and development and nutrition and those other things, but immune status all by itself. If you're immunocompromised because you're getting chemotherapy or immunosuppression for a transplant, those are all going to affect host susceptibility.

02:21 And finally, accessibility. So, I would hazard a guess that almost everyone who is watching this right now is a no way in any danger of contracting Ebola. Just because you're not where Ebola is. So, accessibility, if you are not near the environmental niches, if you are not contacting potential people who have other infections, then you're not likely to be very susceptible. Kind of again a no brainer but something to consider. And then there are the host barriers. So, we're going to talk about these in turn but I just want to put them up front here. So the skin is very important barrier, the respiratory tract has various elements that make it a barrier because we're inhaling millions upon millions of microorganisms everyday. The GI tract has to be another sort of barrier that we'll talk about. The GU tract, genitourinary tract, is also another barrier. And then finally the immune system.