Q:

Why are human feces riskier than urine with regard to spreading diseases?

I came across a Wikipedia article titled "Urine diversion". It explains different ways, including using special diversionary toilets, and treating the collection of human feces and urine separately as opposed to just flushing everything down the toilet, maybe something most people in well developed countries are used to. These sentences caught my attention:

Separate treatment of the two types of waste is justified since urine is nearly sterile and low in pathogens, provided an individual is healthy.[3] This means that urine can be readily utilized as a fertilizer or discharged with less risk to community.[4]
Human feces, on the other hand are high in pathogens, including up to 120 viruses and need to be treated well before it can be safely used in agriculture. 
Urine diversion - principle (Wikipedia article)

So urine is nearly sterile and low in pathogens, however it qualifies that statement with the condition that the person producing that urine is healthy. I assume when they compare this to feces they do so with all other things being equal (ie., provided an individual is healthy).
It claims that there may be up to 120 viruses in feces, as opposed to urine which it characterizes as being relatively low risk. I assume that feces are riskier in transmission of diseases because it naturally contains more pathogens, but it doesn't explain what these viruses or pathogens are. I was wondering if someone could elaborate on the risks of feces over urine, and also, notwithstanding that it says that urine is relatively low-risk, what health issues can be caused by reusing urine.

A:

Many gut microbes are pathobionts or opportunistic pathogens — these organisms can be part of the normal gut flora (microbiome) of healthy individuals, but under the right (for them) circumstances (e.g.s: when introduced into the bloodstream by a cut, or due to immunosuppression) they can cause disease.1,2
As for urine, there are many fewer microbes present (>400 species and 109 to 1011 bacteria/g of contents in the large intestine vs. <100 species and on the order of 105 bacteria per mL of urine).3-5  Note, however, it is still far from sterile!5-7
You might also find the following of interest:
https://www.who.int/water_sanitation_health/dwq/iwachap5.pdf

References:
1: Hornef, M. (2015). Pathogens, commensal symbionts, and pathobionts: discovery and functional effects on the host. ILAR journal, 56(2), 159-162.

2: Price, L. B., Hungate, B. A., Koch, B. J., Davis, G. S., & Liu, C. M. (2017). Colonizing opportunistic pathogens (COPs): the beasts in all of us. PLoS pathogens, 13(8), e1006369.

3: Davis, C. P. (1996). Normal flora. In Medical Microbiology. 4th edition. University of Texas Medical Branch at Galveston.

4: Barnett, B. J., & Stephens, D. S. (1997). Urinary tract infection: an overview. The American journal of the medical sciences, 314(4), 245-249.

5: Lewis, D. A., Brown, R., Williams, J., White, P., Jacobson, S. K., Marchesi, J., & Drake, M. J. (2013). The human urinary microbiome; bacterial DNA in voided urine of asymptomatic adults. Frontiers in cellular and infection microbiology, 3, 41.

6: Hilt, E. E., McKinley, K., Pearce, M. M., Rosenfeld, A. B., Zilliox, M. J., Mueller, E. R., ... & Schreckenberger, P. C. (2014). Urine is not sterile: use of enhanced urine culture techniques to detect resident bacterial flora in the adult female bladder. Journal of clinical microbiology, 52(3), 871-876.

7: Whiteside, S. A., Razvi, H., Dave, S., Reid, G., & Burton, J. P. (2015). The microbiome of the urinary tract—a role beyond infection. Nature Reviews Urology, 12(2), 81.

A:

Urine is contaminated with skin bacteria and possibly something excreted from your blood. A very tiny fraction of urine is microbial.
Feces is basically a solid mass of undigested food, gut bacteria, and their byproducts. Many gut bacteria in particular have evolved in the conditions the human body produces so many can survive well in human tissue. about 30% of the solid weight of feces is bacteria or bacterial byproducts. 
All things being equal, feces contains orders of magnitude more bacteria than urine by unit mass.

A:

The kidney is essentially a filter which extracts dissolved waste products & water from the blood.  It basically allows only molecular-sized things to pass, otherwise it would continually leak blood cells.
The digestive tract OTOH is basically a tube.  Stuff gets mashed up at one end and ejected at the other.  Even fairly large objects can pass through unaltered.  So any bacteria that enter the tract can (if they can survive stomach acids &c) pass through the tract.  Moreover, many find the intestines a warm & comfortable place to live and reproduce.  Once the population has grown, some of it will be ejected with each bowel movement.  While most of the bacteria are harmless or even beneficial (see "gut microbiome" for more info), some, like some varieties of E. coli or the cholera bacterium, can cause serious illness.

