A pond has entry and exit portals. If an organism is small enough, it can exit a pond on wind and spray. Some organisms can ﬂoat downstream to another pond, while birds and insects can simply ﬂy. Larger organisms such as amphibians can hop or walk from pond to pond to lay eggs. And a new exit portal has recently appeared in the form of boats and trailers that carry invasive weeds and animals from one pond and deposit them in another.
Think analogously of the entry and exit portals of an animal. How can pathogens leave one body and enter another? While skin covers at most two square meters in humans, mucus membranes of the respiratory, digestive, and reproductive tracts cover more than 400 square meters. So, mucus membranes become good portals.
Successful diseases can exploit obligate behaviors. Animals, for example, must breathe continually, so exploiting the respiratory pathway—being breathed out into the air by one animal and breathed in by another— is a reliable and ever-present method of transmission.
How infections can leave and enter the body (mix and match):
Portals of exit:
- Droplets (sneezing, coughing)
- Seminal ﬂuids
- Vaginal ﬂuids
- Blisters, boils, zits (via scratching, breaking the skin)
- Blood (mosquitoes, ticks, hemorrhage)
Portals of entry:
- Blood (cuts, wounds, insects, needles)
- Eyes (conjunctivia)
- Mouth (mucosa)
- Mouth (tooth/gingivia junction)
- Skin (scabes, triconosis, warts, etc.)
- Urinary tract
- Vagina (mucosa)
- Penis (mucosa)
Animals must also eat frequently and periodically, so exploiting the oral pathway—leaving through urine and feces and getting back through the alimentary canal—is another reliable path, at least under conditions in the wild or with animals that live on and eat grass (Figure 14.0.1, right).
Animals must reproduce, so exploiting the genital pathway—where many animals come into direct bodily contact— is a third reliable path. This can be especially productive, as mucosal tissues of high surface area are touching and infected fluids can be transferred from one sex to another. A pathogen that can get into seminal fluid of a mammal, for example, has a direct path for transmission.
Finally, many animals care for young, so exploiting parental care can be a fourth reliable path. A pathogen that can enter a mother’s milk has a direct path for infecting her offspring. Such transmission from parent to offspring is called “vertical transmission,” with other forms called “horizontal transmission.”