MS flare-ups are triggered when the immune system launches a misguided attack on the protective coating around nerve fibers in the brain and spinal cord. While the underlying disease process is always present, specific triggers can tip the balance and set off a new episode. The most well-documented triggers include infections, psychological stress, low vitamin D levels, heat exposure, and hormonal shifts after childbirth.
What Happens During a Flare-Up
In MS, immune cells that normally fight off infections cross the blood-brain barrier and enter the central nervous system, where they attack myelin, the insulating layer around nerve fibers. This causes localized inflammation, scarring, and damage to the nerves themselves. A true flare-up (also called a relapse or exacerbation) involves new or worsening neurological symptoms that last at least 24 hours, typically persisting for one to two months before fully or partially resolving. To count as a separate relapse, it needs to occur at least 30 days after the start of a previous one.
The immune cells most involved are a type called CD8+ T cells, which dominate the damaged areas and are linked to permanent neurological deficits. These cells drive inflammation through signaling molecules that amplify the immune response and recruit more cells to the attack site.
Infections Are the Most Common Trigger
Upper respiratory infections, including common colds and flu, are the single most studied trigger for MS relapses. One prospective study found that viral upper respiratory infections carried a relative relapse risk of 2.1, meaning people with MS were roughly twice as likely to have a flare-up in the window from two weeks before to two weeks after their first cold symptoms. The annual attack rate during these infection-associated periods was 5.7 compared to 1.6 during infection-free periods.
Even milder viral infections that wouldn’t concern most people can be enough to activate the immune system in ways that spill over into attacking myelin. The mechanism is straightforward: an infection revs up immune activity broadly, and in someone with MS, that heightened state makes it easier for rogue immune cells to cross into the brain and cause damage. Urinary tract infections and gastrointestinal bugs have also been studied, though the statistical link to relapses is less consistent than with respiratory infections.
Stress and Routine Disruption
Chronic stress and major life disruptions increase the odds of new disease activity. A well-known study by Mohr and colleagues tracked MS patients monthly and found that conflict and disruption in routine increased the odds of developing new brain lesions by 64% in the eight weeks following the stressful period. That’s a meaningful increase in measurable disease activity, though the researchers noted it wasn’t consistent enough to predict clinical symptoms on its own.
The type of stress matters. It’s not a single bad day at work but sustained interpersonal conflict, major life changes, or prolonged disruptions to daily patterns that carry the greatest risk. Stress hormones like cortisol have complex effects on the immune system: short bursts can temporarily suppress inflammation, but chronic stress appears to dysregulate immune function in ways that promote the kind of autoimmune activity seen in MS.
Low Vitamin D Levels
Vitamin D plays a significant role in regulating the immune system, and low blood levels are consistently linked to higher relapse rates. In a Dutch study that tracked relapsing-remitting MS patients for nearly two years, every 10 ng/mL increase in blood vitamin D was associated with a 34% decrease in relapse risk. The researchers estimated that raising vitamin D levels by about 20 ng/mL could cut the risk of a relapse by up to 50%.
People with MS whose vitamin D levels fell below 20 ng/mL had significantly more relapses than those with levels between 20 and 40 ng/mL or above 40 ng/mL. This helps explain why MS relapses are more common in winter months, when sun exposure drops and vitamin D production slows. It also explains part of the well-established geographic pattern where MS is more common farther from the equator.
Heat and the Uhthoff Phenomenon
Heat doesn’t cause new nerve damage, but it can temporarily worsen existing symptoms in a way that closely mimics a true relapse. This is called the Uhthoff phenomenon, and it happens because even a small rise in core body temperature slows electrical signals along nerves that have already lost some of their myelin insulation. Hot baths, exercise, fever, saunas, and hot weather are all common triggers.
The key distinction is that these “pseudo-relapses” typically resolve within 24 hours once body temperature returns to normal. No new damage is occurring. Before MRI technology existed, the hot bath test was actually used as a diagnostic tool for MS. If you notice symptoms flaring in the heat but they clear up with cooling, that’s almost certainly Uhthoff phenomenon rather than a true relapse.
Pregnancy and the Postpartum Period
Pregnancy itself is actually protective. The immune shifts that prevent the body from rejecting the fetus also suppress the type of autoimmune activity that drives MS. Many women experience fewer relapses during pregnancy, especially in the third trimester. The problem comes after delivery, when the immune system rebounds sharply.
Nearly 30% of women with MS experience a relapse within the first three months after giving birth, and almost 50% have one within six months. This postpartum surge is one of the most predictable triggers in MS. The rapid hormonal changes, combined with sleep deprivation and the physical stress of caring for a newborn, create a perfect storm for immune reactivation.
Sleep Disruption
Nearly half of people with MS report moderate to severe sleep problems, including insomnia, restless legs syndrome, and sleep apnea. Poor sleep doesn’t just make fatigue worse. On a biological level, fragmented sleep tilts the immune system toward inflammation by increasing the same signaling molecules that drive MS activity. Sleep is also when the brain clears metabolic waste, and disrupting that process adds cellular stress to an already vulnerable nervous system.
While sleep deprivation hasn’t been studied as rigorously as infections or stress in terms of relapse risk, the immune pathways it activates overlap heavily with those involved in MS flare-ups. Treating underlying sleep disorders is one of the more actionable steps for reducing overall disease activity.
Smoking
Smokers with early MS progress to a confirmed diagnosis faster than nonsmokers. In one study, 75% of smokers developed clinically definite MS within 36 months compared to 51% of nonsmokers, with a hazard ratio of 1.8 for disease progression. Smokers also had a significantly shorter interval to their first relapse. The relationship between ongoing smoking and individual relapses is harder to pin down statistically, but the overall effect on disease progression is clear and consistent across studies.
Vaccines Do Not Increase Relapse Risk
This is a common concern, but the evidence is reassuring. Standard vaccines against diseases like measles, mumps, rubella, and tetanus have not been found to trigger MS onset or worsen the disease. The same holds for COVID-19 vaccines: a large German registry study found no significant change in annualized relapse rate before versus after vaccination (0.109 pre-vaccination compared to 0.116 post-vaccination). When relapses did occur near the time of vaccination, they were more closely related to existing disease activity and treatment status than to the vaccine itself.
Skipping vaccines, on the other hand, leaves you vulnerable to the very infections that are proven to trigger relapses. The risk-benefit calculation strongly favors staying up to date on vaccinations.
How Flare-Ups Are Treated
When a true relapse occurs, the standard treatment is a short course of high-dose corticosteroids, typically taken for three to five consecutive days. This can be done orally or intravenously, and the goal is to reduce inflammation quickly and shorten the duration of the attack. Most relapses resolve fully or partially over weeks, though some leave behind residual symptoms. Corticosteroids speed recovery but don’t appear to change the long-term outcome of any individual relapse.
The more important strategy is prevention through disease-modifying therapies, which work by keeping the immune system from launching those attacks in the first place. These medications reduce relapse rates significantly, and in the German vaccine study, researchers noted that relapses occurring in well-treated patients were more related to gaps in treatment than to any external trigger. Managing the controllable factors, including infections, stress, vitamin D, sleep, and smoking, adds another layer of protection on top of medication.