The word "co-infection" simply means being infected with more than one pathogen at the same time. In the context of tick-borne illness, it refers to other bacteria, parasites, or viruses that may be transmitted alongside Borrelia.
Understanding co-infections is not about assuming you have them. It is about knowing they exist — so you can ask better questions and understand why some presentations are more complex than others.
What "co-infection" means
When a tick feeds, it may transmit more than one microorganism. Each pathogen has its own biology, its own way of affecting the body, and in some cases, its own testing requirements.
This matters because the standard Lyme test — designed to detect Borrelia antibodies — tells you nothing about Bartonella, Babesia, or Ehrlichia. These require separate, specific testing.
A negative Lyme test does not rule out a co-infection. And a positive Lyme result does not confirm that Lyme alone explains the full picture.
Common co-infections you may hear about
These are the most commonly discussed co-infections in the context of tick-borne illness. Each is distinct. None should be self-diagnosed. But knowing they exist — and what systems they tend to affect — can help you have more informed conversations.
- Often called "cat scratch disease" in its more common form
- Associated with neurological and psychiatric symptoms
- Skin streaks (stretch-mark-like) sometimes reported
- Can affect lymph nodes, liver, spleen
- May cause burning or electric sensations
Bartonella can persist and is known for affecting the nervous system in ways that overlap with neurological Lyme. Some practitioners consider it one of the most complex co-infections to address.
- A parasitic infection of red blood cells
- Can cause sweating, chills, and air hunger
- Fatigue that is disproportionately severe
- Night sweats even when not feverish
- Similar in some ways to malaria
Babesia does not respond to antibiotics used for Borrelia. If it is present alongside Lyme and not identified, some people find they do not improve as expected on standard treatment.
- Bacterial infections affecting white blood cells
- Often cause early flu-like illness
- Can reduce white blood cell or platelet counts
- May be more acute than Lyme in early presentation
- Respond to doxycycline (as does early Lyme)
These infections are more commonly recognised acutely. Their chronic or persistent presentations are less studied, but clinically relevant for some patients.
- Rickettsia species transmitted by ticks and other vectors
- Mycoplasma — a cell-wall-deficient bacterium
- Various viral co-infections in some regions
- Research into tick microbiome is still evolving
The landscape of tick-transmitted pathogens is broader than commonly recognised. What is known today is not the full picture of what is possible.
How co-infections can change symptom patterns
When more than one pathogen is active, symptoms can appear in more body systems simultaneously, shift more unpredictably, or be more severe than a single infection would suggest.
This is one reason why some people with Lyme do not respond as expected to standard antibiotic courses — not because the treatment is wrong, but because it may not be addressing the full picture.
Understanding this does not mean every complex case involves co-infections. It means the possibility is worth considering — and worth raising with a clinician familiar with tick-borne illness.
When to think about co-infections
There is no single rule for when to consider co-infections. But the following patterns may be worth raising with your clinician:
Asking about co-infections is not about self-diagnosing. It is about making sure the full landscape is considered — not just the most common or most expected explanation.
A grounded perspective
- Co-infections are real and well-documented — but not universal.
- They require specific testing to identify — not assumed from symptoms alone.
- Understanding them helps you ask targeted questions, not reach conclusions.
- A clinician familiar with tick-borne illness is the right guide here.
Complexity is not a reason to give up. It is a reason to be thorough. And thoroughness begins with knowing what to look for.
Related pages
Symptom patterns across body systems — and how to read clusters rather than isolated events. A useful companion to understanding co-infections.
Standard Lyme tests won't show co-infections. Understanding what different tests look for — and what they can miss — is an important next step.
Healing mentality checkpoint
Learning about co-infections can feel alarming. Try to hold this as "useful context" rather than "more things that might be wrong." Complexity, once named, becomes navigable.
Read about healing mentality →Sources & further reading
- ILADS — co-infection clinical overview: ilads.org
- Horowitz, R. — Why Can't I Get Better? (2013)
- Galaxy Diagnostics — Bartonella research overview
- Sapi E. et al. — Persistence of Borrelia burgdorferi following antibiotic treatment (2012)
Last updated: March 2026