Why does food taste bland when you have a cold?

The tongue detects only five basic tastes: sweet, salty, sour, bitter, and umami. Everything else you experience as flavor comes from olfaction, specifically retronasal smell, where volatile aroma compounds released by food in your mouth travel up the back of the throat to the olfactory receptors. That channel supplies the overwhelming majority of what people call taste, which is why a stuffed nose flattens food to bare sweetness and saltiness. You can demonstrate it by pinching your nose while eating a jellybean, sweetness registers and the fruit flavor disappears until you release.

Congestion blocks that pathway by physically obstructing airflow and by inflaming the nasal lining and coating the olfactory receptors in mucus. Illness compounds it further, since inflammation can temporarily impair taste receptors directly, and some medications alter taste as a side effect. This is also the mechanism behind the smell and taste loss that became widely known during COVID, where the virus affects supporting cells in the olfactory epithelium, and it explains why that loss was so distressing, since it removes most of the pleasure of eating without touching the tongue at all. Recovery usually follows the congestion clearing, and persistent loss of smell after an illness resolves is worth reporting to a doctor, since smell training and evaluation exist for it.