Say you have come down with what you think is food poisoning and tell your boss that you are spending equal amounts of time between bed and the bathroom. They aren't going to want to inquire further, nobody wants to catch it if it's contagious, and nobody wants to be the guy who forced their employee to come into work and thus making that person shit themselves while on the clock. Also not even greedy, compassionless arsehole bosses want to pay a staff member who is going to spend a few hours on the toilet nor do they want to be stuck with the responsibility for a person who cannot leave the workplace because things have deteriorated and now the staff member cannot be more than 10 mins away from a toilet.

If they push you on details, tell them that you're doing okay keeping food down but the rest is not fun and that you aren't so bad that you need to go to the hospital but you aren't in any state to leave the house at the moment.

If you have some bad diarrhoea then you probably aren't going to sound very sick, which is nice for lying about needing the day off. Just don't sound too chipper when you call in. A bout of bad diarrhoea can pass quickly enough and there aren't going to be any outwardly visible indications of lingering sickness that you would expect from someone who, say, called in because of severe hayfever or the flu so this is probably your safest bet.