My understanding is that, as a general rule, a healthy woman may not delay going to the Mikvah for her own (or even her husband's) convenience.
But I'm wondering if there are sources discussing whether a woman should or shouldn't go to Mikvah when she and/or her husband has a non-life threatening illness that may be made worse by going (eg., flu, strep, ear infection)?
Does it matter if either she or her husband may be contagious?
There will be a great deal of particulars depending on the case; I strongly recommend you contact yoatzot.org, who have dealt with dozens (hundreds) of such cases. Deena Zimmerman published an article in Tradition a few years ago specifically addressing mikvah concerns that have arisen in cases of breast cancer, in which all piskei halacha have been validated by Rabbi Yehuda Herzl Henkin, shlit'a.
According to my wife, she has gone with a flu and bronchitis, but wouldn't go with an ear infection because that will definitely make her more sick. She even knows of people who have gone the same day as getting stitches. Also, sometimes a Mikvah lady will not allow a woman to go, if she has a disease or sickness which is contagious.
