I can't understand what is happening

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I have three cats inside. One was sick. After that, the second cat was a bit sick. It was not something serious. Both were taken to the vet. They are both healthy now. And today my third cat got sick too. He has a fever. The vet couldn't find the cause. They were tested and they don't have a serious disease. But it's too much. How can all three be sick one after another? And the vet doesn't know why. Fortunately, they don't have feline leukemia or other severe disease. But it's something contagious. And it's night and I can't take the cat to the vet. I noticed that he seemed sick and didn't eat today, but he didn't have such a high fever. And my second cat who got sick didn't need treatment. When we took her to the vet she was already healthy. The vet decided to avoid giving her any treatment. But I wonder why vets can't find the cause. This happened in the past too with Blue. He had a high fever, but the vet couldn't find the cause. Now it's happening again. They are all rescued cats. They were tested before taking them inside. Vets told us to take them because they are healthy cats. I was almost sure that my third cat would be sick too. And this happened. But I think that it's nothing serious because my two cats are healthy. This is weird. All tests were negative.