About 80 to 90 percent of feline upper respiratory infections are caused by two viruses: feline herpesvirus-1 and feline calicivirus. Less common causes include the bacteria Chlamydia felis, Bordetella, and Mycoplasma. It spreads easily between cats but is not contagious to people.

What are the symptoms of an upper respiratory infection in cats?

Sneezing, runny nose, watery or goopy eyes, conjunctivitis, fever, lethargy, and reduced appetite. Mouth ulcers are a hallmark of calicivirus, and eye and corneal problems are more typical of herpesvirus.

Does cat flu ever go away completely?

The signs usually resolve in a week or two, but herpesvirus does not leave the body. It goes dormant for life and can reactivate during stress, so a recovered cat may flare again. Calicivirus-infected cats can keep shedding the virus for months.

Does the vaccine prevent cat flu?

Not entirely. The core FVRCP vaccine significantly reduces the severity of illness and shedding, but it does not fully prevent infection or stop a cat from becoming a carrier. It is still strongly recommended because it makes the disease far milder.

When should I take a cat with a cold to the vet?

See a vet if your cat stops eating, is struggling to breathe, becomes very lethargic or dehydrated, has a high fever, or is a young kitten, since kittens can deteriorate quickly. Most mild cases recover with supportive care, but those warning signs need attention.

Cat Flu: Feline Upper Respiratory Infection, Herpesvirus, and Calicivirus

Here is the short version: the sneezing, runny-eyed cat cold is one of the most common illnesses in cats, especially kittens and cats from shelters or multi-cat homes, and almost all of it comes down to two viruses. The twist that surprises most owners is that one of those viruses never fully leaves. It hides in the body for life and can flare again whenever the cat is stressed. This guide explains what cat flu is, why it recurs, what the vaccine actually does, and when a runny nose becomes a reason to call the vet.

Two viruses cause almost all of it

Feline upper respiratory infection, often called cat flu, is overwhelmingly viral. Roughly 80 to 90 percent of cases are caused by feline herpesvirus-1 and feline calicivirus.² Less often, bacteria such as Chlamydia felis, Bordetella, or Mycoplasma are involved.¹ It spreads between cats through sneezes, direct contact, and shared bowls, and it is highly contagious in crowded settings, which is why shelters and catteries see so much of it. Reassuringly, it does not spread to people.²

The signs, and a clue to which virus

The picture looks a lot like a human cold: sneezing, nasal and eye discharge, conjunctivitis, fever, lethargy, and a drop in appetite.¹ Two details help point to the culprit. Mouth ulcers on the tongue or palate are characteristic of calicivirus, while eye and corneal problems, including painful corneal ulcers and recurring conjunctivitis, lean toward herpesvirus.³¹ A cat that goes off its food because its nose is blocked or its mouth hurts is common, and it matters, because a cat that stops eating needs help sooner.

Why it comes back: the lifelong carrier

This is the part that defines feline herpesvirus. Once a cat is infected, the virus is there for life. Cornell notes that up to 80 percent of infected cats become latent carriers, and of those, a large share periodically shed the virus again, usually when stressed, by a move, a new pet, boarding, or illness.¹ So a cat that recovered months ago can suddenly have watery eyes and sneezes again, not because it caught something new, but because its own dormant virus reactivated.⁴ Calicivirus works a little differently, with infected cats shedding the virus for months after they seem recovered.²

What the vaccine really does

The core FVRCP vaccine covers both of these viruses, and it is worth getting, but it is important to be clear-eyed about what it does. It significantly reduces the severity of illness and the amount of virus a cat sheds, yet it does not fully prevent infection and does not stop a cat from becoming a carrier.¹ That is still a strong reason to vaccinate, because a vaccinated cat that does get infected tends to be far less sick. It just is not a guarantee against ever catching cat flu.

How it is treated

For most cats, treatment is supportive care rather than a cure: keeping the cat eating and hydrated, gently wiping away eye and nose discharge, and good nursing at home.¹ Antivirals can help in specific herpesvirus cases, particularly eye involvement, and antibiotics are used only when a secondary bacterial infection sets in, not for the virus itself.¹ One widely sold remedy worth a caveat: L-lysine supplementation is popular but controversial, and several studies suggest it does not help and may even make shedding worse, so it is not a reliable fix.¹

See a vet promptly if your cat stops eating, is breathing with effort, becomes very lethargic or dehydrated, runs a high fever, or is a young kitten, since kittens can go downhill fast. Those are the cases where a "cat cold" needs more than nursing.

What it costs

A routine case treated with supportive care is not especially expensive. In one insurer's 2025 claims report, a feline respiratory infection averaged about $534 in the first month of care, and it ranked among the ten most common cat conditions.⁵ Costs rise with complications, hospitalization for a cat that will not eat, or chronic herpes eye disease. These are typical figures that vary by region and severity.

What to do this week

Keep your cat's FVRCP vaccine up to date, knowing it softens the illness even though it cannot fully prevent it.
If you have a recovered cat that flares when stressed, plan ahead for big changes (moves, new pets, boarding) to reduce reactivation.
For a cat with the sniffles, watch the appetite above all, and call the vet if it stops eating or struggles to breathe.
In a multi-cat home, isolate a sneezing cat where practical and wash hands and bowls to limit spread.

Cat flu is common, usually mild, and rarely dangerous in an otherwise-healthy adult cat, which is the reassuring headline. The wrinkle to remember is that herpesvirus is a lifelong houseguest that resurfaces under stress, so managing your cat's stress is as much a part of long-term control as any medication. Watch the appetite, keep the vaccine current, and most cats weather their colds just fine.