- The test result was negative and it said she caught a ordinal cold.
- Fortunately, the test results were negative. They said she caught a ordinal the common cold. ("ordinary cold" is fine, but "the common cold" is a very common English expression. When talking about the medial exams, it is implied that "they" means medical professionals, whether doctors, nurses, technicians, etc. If you are talking about a specific doctor you can use "he" or "she". But, even then it is OK to use "they" in a vague way.)
- I made an entry into an 100km marathon race!
- I entered a 100km marathon race!If you sign up for a 100km race, then you say you entered it. If you were selected from a bunch of people to participate, then you say you "qualified for a 100km race."