I applied for an IB internship with CS in London a few weeks before Christmas. Two days ago I received an email inviting me to tests. At the time I was quite ill and the email did not disclose the deadline for completing the tests. I sent a short and respectful email to HR saying that I am ill and asked how many days I have to complete the tests.
2 hours later I received a rejection email.
Moral of the story? Assume HR are gods and don't give them any reason to ding you. And if you are sick, just do the tests.