Multiple sclerosis does make you more susceptible to certain illnesses, though the reasons are more layered than most people expect. Some of the increased risk comes from the disease itself, some from the medications used to treat it, and some from the physical complications MS causes over time. Understanding which factors apply to you can help you take practical steps to stay healthier.
How MS Changes Your Immune System
MS is fundamentally a disease of immune dysregulation. Your immune system isn’t simply weakened; it’s misdirected. In people with MS, the immune system shows broad overactivation of certain pathways, including inflammatory signaling, antiviral responses, and immune cell migration. This chronic state of activation means your immune resources are, in a sense, busy fighting the wrong battle.
Epstein-Barr virus, which most people carry quietly, appears to play an ongoing role in this process. In people with MS, reactivation of the virus can reshape the immune landscape, maintaining persistent immune activation that keeps antiviral pathways fired up and promotes inflammation. The result is an immune system that’s simultaneously overactive against your own nervous system and potentially less efficient at responding to new threats in a coordinated way.
MS Medications and Infection Risk
The treatments that control MS work by dialing down parts of the immune system, and each class of medication creates a distinct pattern of vulnerability. This is often the biggest factor in infection susceptibility for people with MS.
B-cell depleting therapies (like ocrelizumab and rituximab) carry the highest risk of serious infections requiring hospitalization. These drugs target the immune cells responsible for producing antibodies, which leaves you more vulnerable to bacterial infections like listeriosis and tuberculosis, as well as a fungal lung infection called Pneumocystis pneumonia.
Medications that trap immune cells in your lymph nodes (like fingolimod) have been linked to certain viral infections, including shingles and, in rare cases, a serious brain infection called PML. Natalizumab, which prevents immune cells from crossing into the brain, also carries a PML risk that increases with longer use, particularly beyond two years of treatment.
The practical takeaway: your specific medication matters more than your MS diagnosis alone when it comes to infection risk. If you’re on a B-cell depleting therapy, you’re in a meaningfully different risk category than someone on a milder treatment.
Vaccines May Work Differently for You
One of the less obvious ways MS treatment affects your defenses is through reduced vaccine effectiveness. People on B-cell depleting therapies produce significantly fewer antibodies after vaccination compared to healthy individuals. In studies of COVID-19 mRNA vaccines, people on ocrelizumab showed lower levels of virus-specific antibodies, and some produced no detectable antibodies at all after their initial vaccination series. Antibody levels did increase after booster doses and breakthrough infections, but remained below what healthy people typically achieve.
The timing of your infusion relative to vaccination matters. The more B cells you have circulating when you get vaccinated, the better your antibody response tends to be. Your neurologist can help you time vaccines to fall at the right point in your treatment cycle. One reassuring finding: T-cell responses to vaccines appear to remain comparable to those of healthy people, even on B-cell depleting therapy. This means your immune system retains some protective capacity even when antibody production is blunted.
Urinary Tract Infections Are Especially Common
UTIs are one of the three most common non-neurological complications of MS, and the reason is largely mechanical. MS frequently damages the nerves controlling the bladder, leading to incomplete emptying, urinary retention, and stasis. Urine that sits in the bladder becomes a breeding ground for bacteria, most commonly E. coli, Pseudomonas, and Klebsiella.
As the disease progresses, bladder dysfunction tends to worsen. Many people eventually need intermittent self-catheterization or a permanent catheter, both of which introduce additional infection risk. Catheters provide a surface for multiple types of bacteria to colonize, and even without obvious symptoms, chronic bladder irritation from urinary stasis can cause ongoing low-grade infection. Left untreated, recurrent UTIs can escalate to systemic infection and sepsis. The hospitalization rate and mortality rate from UTIs in people with MS is notably higher than in the general population, making bladder management a genuinely important part of staying healthy.
Respiratory Infections and Advanced MS
In more advanced MS, respiratory vulnerability becomes a serious concern. Demyelination in the brainstem and cervical spinal cord can weaken the muscles you use to breathe and cough. The correlation is direct: the more severe the disease, the greater the reduction in respiratory muscle strength.
Weak expiratory muscles mean you can’t cough effectively, which makes it harder to clear mucus and bacteria from your lungs. This raises the risk of pneumonia significantly. Respiratory infection-related mortality in people with MS is twice that of the general population. Reduced mobility compounds the problem, since spending more time seated or lying down further limits lung expansion and clearance.
Why Common Illnesses Feel Worse With MS
Even when an infection isn’t dangerous on its own, it can feel dramatically worse when you have MS. This happens through a phenomenon called pseudo-relapse, where a fever, flu, UTI, or other systemic illness temporarily worsens your existing MS symptoms. You might experience a sudden return of old neurological symptoms, increased fatigue, or noticeably worse mobility. This isn’t new damage to your nervous system. It’s your already-compromised nerve pathways functioning even less efficiently under the stress of illness. The symptoms typically improve once the infection clears.
The distinction between a true relapse and a pseudo-relapse matters because the treatments are different. A true relapse involves new inflammatory activity and may call for steroids, while a pseudo-relapse resolves by treating the underlying infection. Both pseudo-relapses and actual relapses occur at higher rates during infections, meaning illness can trigger either temporary symptom flares or genuine new disease activity.
Steroid Treatments During Relapses
When you do have a true MS relapse, the standard treatment is a short course of high-dose intravenous steroids. Steroids suppress inflammation by reducing immune cell activity and lowering levels of key inflammatory proteins. This helps resolve relapse symptoms, but it also temporarily weakens your defenses. Long-term or repeated steroid use clearly increases infection risk, and high doses (the kind used for MS relapses) can prolong recovery from concurrent infections. Some clinical guidelines now recommend avoiding steroids or using the lowest effective dose for mild relapses, partly to limit this window of vulnerability.
That said, a single short course of steroids (three to five days) does not appear to dramatically increase your risk of catching a new infection. The concern is more relevant for people who experience frequent relapses requiring repeated steroid pulses.
Practical Ways to Reduce Your Risk
Your infection risk profile depends on your specific combination of disease severity, medication, and physical complications. A few things consistently help:
- Bladder management: If you have any urinary symptoms, proactive management with pelvic floor exercises or timed voiding reduces UTI frequency. If you self-catheterize, strict hygiene technique is critical.
- Vaccine timing: Work with your neurologist to schedule vaccines when your immune system is best positioned to respond, particularly if you’re on B-cell depleting therapy.
- Respiratory strength: Breathing exercises and physical activity help maintain respiratory muscle function, which directly lowers pneumonia risk.
- Infection awareness: Know that a worsening of your MS symptoms may signal an underlying infection, especially a UTI, even before you notice typical infection symptoms like pain or fever.