Per my comment to the question, here is an answer to the same question asked on ResearchGate:
Whole genome sequencing of human tissue samples often results in reads aligning to bacterial references, and this is actually a method used in the diagnoses of infectious diseases.
Understanding the Promises and Hurdles of Metagenomic Next-Generation ...
This seems to be one of rare actual photos of lymph online (suggested in the comment above):
Image: Lymph (source: IJCRI - Case Reports Journal)
This particular lymph (or "lymph fluid" or "lymphatic fluid") was aspirated from a "lymphocele", which is an abnormal collection of lymph in the body (appearing outside of the normal lymphatic system).
While a ...