I have a cold. I have the flu. -- Right now, I am ill.
I get/catch a cold. -- This is not a likely sentence without more information, like "I get/catch a cold every time I fly" to mean that it happens habitually.
I get the flu. -- Also not likely unless you say why it's recurring/habitual, e.g., "Even though I get a flu shot every year, every winter, I still get the flu."
1. This is correct. 2. This is very ungrammatical. You'd need to say more, like: I get a cold off and on. 3. This is very ungrammatical. You'd need to say more, like: I catch a cold off and on. 4. This is correct. 5. This is very ungrammatical. You'd need to say more, like: I get the flu off and on. 6. This is very ungrammatical. You'd need to say more, like: I ca