Brief Answer:
Protozoa/parasites not the only infectious agents

Detailed Answer:
Hello,

Thanks for posting on XXXXXXX

If you are presenting with those symptoms after coming from such a zone such as Mexico, then it is safe to assume that you must have contracted an infection which must not necessarily be parasitic or protozoal in origin but they are numerous other microbes that could cause same symptoms and can not be seen in some stool tests unless of course specific tests for each suspected microbe is done and for that, a laboratory equipped for that should be considered.
Generally even without using the laboratory I usually effective treat such conditions with antibiotics such as metronidazole, ciprofloxacine/norfloxacin, cotrimoxazole/bactrim etc (you will likely require a prescription for that) then you need to avoid certain food regimens that could promote bacteria growth such as sugar-rich meals and fatty meals. Drink much fluids (especially water) and avoid beer/alcohol until your symptoms have cleared off.
Other microbes that could cause these kinds of symptoms are fungi, E. coli, viruses etc.

Hope this is useful enough information and please feel free to write back if you have any follow-up queries.

Dr. Nsah

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