How to know if your cat is really sick?

If you've ever been around a sick cat, you know how stressful it can be. You want to help them feel better, but without knowing what's wrong or how to treat it, it's hard to know where to start! Fortunately, there are some easy ways for pet owners—and even those who don't own pets—to tell if their feline friends are sick.
Changes in appetite or thirst:
If your cat is not eating or drinking, this is a serious sign. Your cat may be dehydrated and need to be seen by the vet immediately.
Changes in appearance:
Weight loss or weight gain, changes in appearance can be a sign of illness. You should be able to feel the cat's ribs and spine. If you can’t, it could be an indicator that your pet has a serious illness or injury.
If you notice any changes in your cat's behavior, such as lethargy or loss of appetite, seek medical help immediately.
The cat spends most of the time hiding:
Cats are notorious for hiding away and being elusive, but it could also be a sign of illness. When your cat is hiding most of the time, this means that something is bothering him or her. It's not uncommon for cats to hide when they're sick—they may be fearful or uncomfortable with their surroundings.
The cat is avoiding social interaction:
If your cat is normally friendly and playful, but suddenly seems to be avoiding you or other cats, it could be a sign of illness. Your veterinarian will likely ask you questions about how often the cat interacts with other pets in their household, including dogs and other cats. The vet will also want to know if there have been any changes in behavior since this sudden change began—for example, if they've started hiding under furniture instead of playing with toys on their owner's desk or counter tops. The vet may also perform tests on the animal (such as blood work) as part of an examination process that includes looking at its eyes for signs of disease such as pink eye or conjunctivitis (pink eye).
Your cat is grooming more frequently or excessively:
Grooming is a way for cats to clean themselves. Cats may groom themselves when they are sick, because it helps them feel better and reduces stress.Excessive grooming can be an indication of underlying illness, such as fleas or parasites.
Your cat's litter box habits change:
Cats are very picky about their litter boxes, and if they start avoiding them (or using them less often), there could be a problem. If your cat is using the box more often, there is probably a health issue. If he or she is using the box less often and seems to not care anymore, this could mean that something is wrong with him or her physically or mentally—but it can also just mean he or she doesn't like what you're doing with his/her bathroom anymore!
Your cat is sleeping more than usual:
Cats can sleep up to 16 hours a day, but if they're not feeling well or have been exposed to something dangerous in the environment (like fleas), then they might need more rest than usual. If this is happening and you don't think your cat should be over-sleeping: Take them to the vet immediately!
Cats can get sick, just like humans. It's important to recognize the immediate signs apart from the aforementioned, which take some time to notice, you can provide care as soon as possible.
Immediate signs of illness in include:
-High fever
-Coughing or sneezing
-Frequent vomiting
-Drop in energy levels
-Hair loss
If you pay attention and be aware of these possible signs, you can easily catch a cat sick before it gets worse and possibly cause an outbreak at home!