Today I am going to discuss a pretty advanced concept in ecological epidemiology. Not only is it advanced, but it’s controversial, even professional scientists argue over these definitions in the literature to this day. Before I scare you off from this blog post however, this concept is essential to understanding why some diseases spread faster than others, why some pathogens cause extinction events, and most importantly why prevention strategies vary depending on the nature of the ailment. Scientists propose that there are two main ways diseases are transmitted through out populations, density dependent disease transmission and frequency dependent disease transmission.
Put very simply (with the complicated math not included), density-dependent transmission is disease transmission “in which the rate of contact between susceptible hosts and the source of new infections increases with host density”, where as frequency-dependent transmission is disease transmission “in which the rate of contact between susceptible hosts and the source of new infections is independent of host density” (Ecological Epidemiology, Michael Begon). In Population Ecology: A Unified Study of Animals and Plants by Michael Begon he specifically talks about this concept in terms of parasites, in fact this super useful blog post also uses parasites when discussing these modes of transmission, however, this distinction can be applied to all types of disease that are viral, bacterial… or even magical in nature.
Speaking of magic, let’s break this concept down so we can better understand it. In Dungeons and Dragons there are a ton diseases that players can be infected with (I’ve picked 12): ‘Blinding Sickness’, ‘Cackle Fever’, ‘Cascade Flu’, ‘Cerebral Parasites’, ‘Demon Fever’, ‘Devil Chills’, ‘Filth Fever’, ‘Mindfire’, ‘Mummy Rot ‘, ‘Red Ache, ‘The Shakes’, and ‘Slimy Doom’… (Seriously, why can’t real disease have these cool names? Ebola? Doesn’t sound bad ass at all.) Based on ‘Infection DC’ (Difficulty Class), ‘Incubation’, ‘Damage’, and general descriptions of symptoms, we can sort all 12 of these into the two transmission types categories.
But before we go any further let’s get some pesky definitions out of the way:
Contact: when an uninfected individual and an infected individual interact.
Effective contact: when an infected person successfully makes an uninfected individual sick
Contact rate: how many times this effective contact occurs over a given time period.
DD transmission: contact rate increases as population density increases and vice versa.
FD transmission: contact rate is independent of population density.
The flu is a great example of a disease that follows DD transmission, the more people there are around you, the more contact you have, the greater chance there is for an effective contact and thus the contact rate is higher. STD’s however, are a bit different. Being around someone with an STD doesn’t automatically open you up to risk of infection, in order for there to be a contact event two individuals (one diseased and one uninfected) must engage in a specific interaction (in this case unprotected sex). Even if there is a ‘contact event’ in the form of unprotected sex it does not guarantee effective contact (the uninfected individual also becoming infected with an STD). Even if there are more people around you it doesn’t mean you are more likely to have unprotected sex (an effective contact event) with them than you would if there were less people around you.
Making more sense yet? Well, let’s break down the D&D diseases and we will see if it gets any clearer…
If you notice diseases transmitted through inhalation and contact tend to be density dependent, and diseases transmitted through injury and ingestion tend to be frequency dependent. This makes sense, as inhalation or contact are very general types of contact events (not like STD’s where unprotected sex is needed), so if more people are present in a population, more people will be infected with the disease. If everyone has equal chance of being exposed, then the contact rate will increase as population density increases. However, injury and ingestion are specific contact events (you have to be engaged in behavior which could result in harm, or drinking/eating something potentially infected), so if there are more people in a population it still doesn’t guarantee there will be more knights drinking tainted water, thus the population density won’t relate to the contact rate at all.
Well, I hope that helped. I also hope I didn’t piss off any D&D hardcore fans, in my experience, they’re the scariest group.